Conversion is a big plus in my mind. For Sig & Glock it's a barrel swap.
Plus if I could only have one caliber of firearms, in pistol rounds, .35 caliber would cover everything from all the 9mms, .38 Spec, .357 Mag, .38 Super, .357 Sig, and everything else I'm not thinking of right now. With some care, you could load the same bullet into alll those rounds. Might not get the best performance, but it could be done. Makes it easier to keep track of reloading supplies.