It gets down to a few factors:
1) Support - Can you get support from the dB supplier, can you get support contracts. Companys will go wikiwakeywoo dB if it has support contracts to back it.
2) Scalability - will it scale for the future.
3) Portability - can we change hardware or are we going to be locked in
4) Cost comparision to the prefered dB, is it competative factoring in all potentual issues/costs. Think banks that outsource and then mess up for a week, those kind of things.
5) Reliability, does it dance the 5 9's that managers understand and not alot else
6) Development cost compared to prefered dB
Best way is to fistly get all your database interactions done via a wrapper, then you are in a better position to change your backend database.
It's a uphill battle and you also have to ask, what is in it for you beyond a good geek-out. Will you gain from this fiscialy as well as the company and if not then you need to think about things like workbombing yourself and stictiching yourself up for a thanks if you suceed and a roasting upto dismissal if you fail.
Change is good, but make sure your being compensated for it or you are doing yourself no favours sadly in the coporate world :(.
But getting all dB interactions done via a wrapper would be a good start for many things and in itself very useful.
Good luck, you will need it sadly enough.
