I am not all that familiar wth the 3.3 maxx, but I have been running cvds on my maxx for many years with no trouble at all. I would highly recommend them.
As I've been researching for my new 3.3, I think the reason Your LHS told you this is because the new 3.3 (4908 version) has a more "beefy" drive train than the older versions, which tended to have problems(?) in that area. In all my searching, it seems unless you do a serious motor upgrade, the stock should handle things just fine provided you're letting the slipper do at least a fraction of it's job.
My plan, as with everything with my new 3.3, is to use it till it breakes. Then it's time to upgrade.
Oh, and what does it do? I think the main "upgrade" to CVD's is that they are steel or aluminum drive shafts... the stock shafts are plastic. (Self explanitory.)
i upgraded to a 3.3 on my t-maxx and went through four or five sets of stock drives went cvds not a problem since had a few good wrecks and jumps not a problem!!!!!!
then new (forward only) t maxx 3.3 have much beefier center axles. the plasctic u-jointa are bigger and the transmission outdrive are a bigger diameter. the early 3.3 maxx has smaller ones.
i found this out after trying to to use a longer new 3.3. chassis on my early maxx 3.3 it works fine but the newer extended axles are not compatible with my trans.
so its either a revo trany(revo tranny has the larger outdrives) or possibly just the revo outdrive shaft installed into my 3.3. chassis.
or just a set of extend aftermarket ones.
thats weird your hobbie shop doesn't try to offer you a aftermarket part. my shop is kinda like that and it sucks. i went in yesterday and asked for a cx2 part and the guy say"we really dont promote upgrading them becuase they are just a trainer"
wtf?
i upgraded to a 3.3 on my t-maxx and went through four or five sets of stock drives went cvds not a problem since had a few good wrecks and jumps not a problem!!!!!!