i hope that the reason that the ambassador class was left out of star trek online was because starfleet felt that the ambassador was 80 years old and 85 years old for the prototype uss nx/ncc ambassador need a serious major retrofit and complete rebuilding from the frame up including the stranghing the frames of the ambassador class. also at the same time star fleet needed to deside weather or not to have one look for the ambassador class or have several sub types.
The real reason was actually stated a while back. Basically, Cryptic thought the profile of the Ambassador was too similar to the Galaxy without belonging on the same Tier.
But players want it, so they're tossing it in. I honestly agree with Cryptic on this, though... the Ambassador was kind of an ugly ship. It wasn't really designed to do anything except look like a less-advanced ship than the Galaxy. It did that job really well in Yesterday's Enterprise, but you never actually saw it on the show as much as other ships, like the Miranda or the Excelsior.
I don't understand why people want it, aside from the "Collect 'em all" mentality, which is a poor reason in my opinion.