thats about it
however without it the bottle is very hard to screw off, hence excess wear to any ASA and the threads on the bottle. plus it can cause the oring to get damaged = leaks
all in all the planet OOPS and rail is good value for money (CP direct mount is £30 i think) and works a treat. having said that the CP is decent and i think it purges also (my brothers 05 CP does anyway)
its personal choice.
bear in mind the CP direct will mount your bottle half an inch or so further back but will lower the height of your hopper/setup about a centimeter too
I have used both and you cant go wrong with either but with the purge system and being smaller the oops from planet wins for me. (plus it was free with my ego!) Had a prob with the allen heads rounding off on the cp rail too, May have just been me over tightening it though! But like lovetone said its what feels good with your set up. If anyone you know has one just steal it for a game or two and see what you prefer.