are you talking about mainstage? using a ms2 in ms3? If so I've done it, not really had any issues. However I do like making new templates so I change my template look every few months or everytime I use a different controller with my gear.
As far as cons I had none at all.
Right, same with Windows beginning with Windows7. Then, it was available for 32 or 64. Folks who didn't know the difference couldn't understand why, even though they loaded in tons of ram on their motherboard, it only accessed/used a maximum of 4 gigs. 64 bit allows you to use much much more.