I've actually noticed the opposite. I see a lot of powertecs, but not many mercs. I see a lot of marauder/sents, followed by sorcerers, then jugg/guardians
Don't see many troopers at all, particularly vanguards, or many consulars, particularly shadows. Ops are rarer than snipers, but neither are common.
The funny thing is all the servers Ive been on have different classes that are played more. For example I was on one server were sent. were everywhere. On another server there is not a lot of sent. and a ton of guardians. Its like that every time I switch servers or am on different servers.