I've worked and been in many studio's in Nashville, and they do not use cheap headphones. Although they do not use audiophile headphones either. Popular choices are:
AKG K240 (some K280)
Fostex T50RP (I saw these even on an episode of "Nashville" and they are quite popular around here)
Also, newer additions you might see Shure SRH440.
Some musicians might bring in some isolation heaphones, or their own pair of Beyers or even Ultrasone's. Most of the time, unless the musician/artist really cares, they use what the studio provides.
Most of the time singers will use one ear one, one ear off to hear. Singers are also in an isolation room/booth or in the studio singing back to a track. Some acoustic musicians too (fiddle, mandolin, chello, etc.). Drummers, bass players, guitar players, mostly wear the cans at full of nearly full blast. One studio in town even uses Crown DC-75's as headphone amps for the loudest possible cans.
I would say that IEM's are used, if someone brings them in. Many already have fitted custom ones for going out on the road for monitoring. Drummers I would think would be most likely to use them for fear of loud cans and bleed-over from the drum mics. (I am a drummer, and yes it happens to me). The worst is when the click is so loud in the drummers headphones it can be heard in the drum overheads.
I would think the biggest problem with using IEM, especially custom molded IEM's in the studio is the constant in and out that you do. It is not like they are in there for hours going through takes. Those guys take breaks in between takes all the time. Headphones totally win in this category. You would not want to shove something in your ear and then yank it out all day long.
I know this is an over-explanation of the subject, but there ya go.
PS - there might be more headphones that are used in the studio that you may have seen or know of, but the ones I listed are the most popular that you will find. This can also be confirmed by going to sweetwater.com to the headphone section and ranking by "Most Popular." The only ones intersected in are the cheap Beringers. And of course, they don't sell Fostex.