Yes of course other convictions air dishonesty, such as fraud, deception etc. But you highlighted 'Theft' which is why I mentioned this. Equally though I would decline fraud and deception. I wouldn't even want to employ, work with or have any personal dealings with such people.
You say that insurers whom accept convicted criminals find the claims book balanced. Good for them. For some, however, the risk isn't worth taking until such time that time has passed that it is unspent. I don't think this is unfair. What IS unfair are those depriving others of property which belongs to them or defraud companies which has a negative effect on those law abiding people who choose not to commit an offence by raised insurance premiums where insurers are left to pick up the pieces.
Criminals shouldn't expect to have the same privileges as those honest people trying to earn a decent living. It's not supposed to be easy otherwise people would not think twice about offending. If you have a problem with the period of time in which the insurers base their assessment then I suggest you take the Rehabilition of Offenders to the Government.