I personally use the glass carrier for 35 mm, although I recently acquired a glassless one that I intend to file the edges of a bit in order to get some of the film rebate in--I also picked up a 6x6 cm version for this same purpose.
The glass carrier is a bit frustrating--it does indeed keep your negatives nice and flat, but I find it very difficult to keep mine free of dust specks. I also had to replace the upper glass piece with the anti-Newton glass, as I was often getting Newton rings in prints before that. If you do intend to use the glass carrier--and it is convenient and very useful in the main--I would highly recommend that modification to it. B&H carries the replacement glass for around $35.