I'm almost positive that many if not all used the new gun they have just unlocked just to get a feel for it. Once I unlock every gun for a specific class, I use the gun I'm most comfortable with. I usually just use random guns trying to get some unlocks in the majority of games, but if I realize my team is in danger of losing, I switch to my best gun which is the AEK-971 for the Assault or the A91 for the Engineer.
I prefer the SV98 because I can't use semi automatic snipers at all. I don't benefit from the faster fire rate because I can never get shots off accurately after the recoil from the first shot. Also, I seem to get more 1 shot kills at chest level.
Also, I prefer automatic guns over others that only use burst/single. If needed, you could switch your gun manually into a different firing mode. I don't bother with that as I could just as easily control my own gun in automatic mode by using controlled bursts.
EDIT: Oh ****! I just noticed I'm about top 5% on PS3! I just need to start playing again.

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