Is the concept of a wormhole a practical one? If not, then what did the scientists do to theorize it? Does it have any limitations pertaining to the speed of light?
Wormholes are hypothetical but they have been theorized as topological fluctuations in spacetime. Simply put, they are exotic but occur as solutions to the Einstein equations in GR. As far as a definition goes, the book Lorentzian Manifolds states:
If a Minkowski spacetime contains a compact region $\Omega$, and if the topology of $\Omega$ is of the form $\Omega$ ~ R x $\Sigma$, where Σ is a three-manifold of the nontrivial topology, whose boundary has topology of the form ∂$\Sigma$ ~ S2, and if, furthermore, the hypersurfaces $\Sigma$ are all spacelike, then the region Ω contains a quasipermanent intra-universe wormhole.
"..Schwarzschild wormhole which would be present in the Schwarzschild metric describing an eternal black hole, but it was found that this type of wormhole would collapse too quickly for anything to cross from one end to the other. Wormholes which could actually be crossed in both directions, known as traversable wormholes, would only be possible if exotic matter with negative energy density could be used to stabilize them... "
The impossibility of faster-than-light relative speed only applies locally. Wormholes allow superluminal (faster-than-light) travel by ensuring that the speed of light is not exceeded locally at any time.
There is an instanton for the production of charged black holes in an electric field, connected by a wormhole. This means that in principle such configurations are produced, at a certain rate that depends on the charge and the electric field. You can think of this as the Schwinger effect for pair creation of strings, extrapolated to the general relativity regime where the strings become black holes. The recent paper "Cool horizons for entangled black holes" by Maldacena and Susskind has a nice interpretation of these wormholes as entanglement.