It's possible that a game may check that this folder exists as part of copy protection but I personally haven't had any that don't work when I remove it. If you do have problems you can always use UMDGen to relink the data.bin and eboot.bin to the param.sfo anyway. This way it won't the update can't run and the two bigger files will take up no room at all.
This is the one I had problem with the UPDATE folder. Try to get rid of it with UMDGen 3.02 and then the game don't work any more. Haven't try 3.50 yet
I've had some problems with some games and removing the update files. I don't remember offhand which ones, but it seemed to be only the one particular .iso file.
I have always deleted the UPDATE folder for all my iso and there isn't any problem at all. but leave the files from the SYSDIR folder. hope this helps.