Earth's magnetic field shields us... from what threat? This is a nice artistic picture of the magnet field of the earth, shielding us from the solar wind. (source)

Wherever you look, it is stated that without our shield, life would not be possible on earth. The movie "The Core" exaggerates this and shows a gamma-laser-ray melting the Golden Gate Bridge, when the core of the earth stops rotating and the field collapses.
So, we should be very lucky to have the magnetic field protecting us. Shouldn't we?
Wikipedia states that solar wind consists mainly of protons, electrons and alpha particles, in the energy range of 1.5-10keV. 
Later, sporadic velocities in a range right below 1000km/s are mentioned, which are still in that energy range.
Alpha particles of that energy have a range of not more than 10cm in air (at ground level). Protons and electrons have a larger range, but still, it's not more than a few meters. So, the air above our heads should be sufficient to protect us from solar wind.
Also, some of the solar wind reaches our atmosphere and generated colourful polar lights, but there seems to be no concern about their hazardousness.
It seems that solar wind is too weak to reach the ground. The energy range also is too low to generate radioactive isotopes.
Cosmic radiation of much higher energy isn't shielded, but even there, we just get some muons (and neutrinos) down here.
So, what's the threat the magnetic field protects us from?
 A: Is it the threat of having to eat five servings of fruits and vegetables a day? 
When the magnetic field deflects charged particles the energy is deflected and this means less radiation and less ionizing radiation from he particles that get deflected.
A solar electron, proton, or alpha particle with 10keV would be capable of ionizing molecules and atoms. And those that it does can then ionize others and so on until it is spread to enough things that it is not ionizing anymore.
In people, ionizing radiation can help form free radicals. And yes, this is natural. But to counter those effects you need to have antioxidants. The more ionizing radiation the more antioxidants you need to deal with them.
A similar thing happens at altitude. It isn't the end of the world but you do suffer more damage. Some of that damage can be countered by just having enough antioxidants, but that means you can't skimp on them as much and get away with it. And some damage just won't be repairable since it is random.
So to me it would be similar to having less atmosphere. That said, not all life is equally vulnerable. Maybe it would be worse for lifeforms of different epochs. But the magnetic field does get pretty weak right as it flips so it can't be that everything immediately dies without the magnetic field to protect it.
I'm sure people have studied exactly how it will affect astronauts on their way to Mars for instance. But that would be human centric. I wasn't sure which things on earth you were worried about protecting.
And if you had higher mutations from more damage to DNA and mitochondria this can be good or bad. We've evolved in an environment with a certain amount of radiation so are adapted to it, more or less might have an impact but if it had always been more or always been less maybe we would be adapted to that and that would be normal.
A: Actually, in a large solar flare particle energies can get up to 1 GeV, but the top energy of some particles is not really the issue.  The issue is the flux of these high energy particles.  A 10 MeV proton or electron pretty much rips through most spacecraft bodies, thus, their electronics are effectively exposed to particles at these energies.
The often associated coronal mass ejections (CMEs) produced in association with large solar flares carry with them enhanced fluxes of >MeV protons and electrons.  These blobs of plasma and magnetic fields compress the Earth's magnetic field, which can induce DC currents in our power grids and expose geosynchronous (or GPS, I forget which orbit at the co-rotating altitude) spacecraft to the high levels of radiation.  After the CME has passed, the effects are not over as they often induce a geomagnetic storm, which enhances the radiation belts and thus further exposes co-rotating spacecraft to high energy particles (thus the name "killer electrons" for the outer radiation belts).
I will add more later and include some links, but the point is that our magnetic field does a tremendous amount to prevent our lives from becoming incredibly complicated, as Timaeus eluded to.
Updated Version
Actually, in a large solar flare particle energies can get up to 1 GeV, but the top energy of some particles is not really the issue.  The issue is the flux of these high energy particles.  A 10 MeV proton or electron pretty much rips through most spacecraft bodies, thus, their electronics are effectively exposed to particles at these energies.
The often associated coronal mass ejections (CMEs) produced in association with large solar flares carry with them enhanced fluxes of >MeV protons and electrons. These blobs of plasma and magnetic fields compress the Earth's magnetic field, which can induce DC currents in our power grids and expose geosynchronous (or GPS, I forget which orbit at the co-rotating altitude) spacecraft to the high levels of radiation. After the CME has passed, the effects are not over as they often induce a geomagnetic storm, which enhances the radiation belts and thus further exposes co-rotating spacecraft to high energy particles (thus the name "killer electrons" for the outer radiation belts).
The Earth's magnetic field also helps protect our atmosphere from ionizing erosion.  By that I mean that once an atom is ionized and exposed to the bulk flow of the solar wind, it will experience a conductive electric field ($\mathbf{E} = -\mathbf{V} \times \mathbf{B}$) and react like a pick-up ion.  The force on the particle from such an electric field can easily exceed the gravitational force, thus freeing the particle from the atmosphere.  Without the Earth's magnetic field, the ionized part of the upper atmosphere, called the ionosphere, would increase due to the addition of the solar wind's ionization effects.  Currently, only charged particles with energies >10-100 MeV, neutral neutrons, or high energy photons (e.g., UV, X-rays, and/or $\gamma$-rays) are able to reach our atmosphere and contribute to the overall ionization.
It is doubtful that during a pole flip of the Earth's magnetic field that we would completely lose our atmosphere, considering several pole flips have happened in the past.  However, the point is that our magnetic field does a tremendous amount to prevent our lives from becoming incredibly complicated, as Timaeus eluded to.
