they also recommend them for Baker. usually to prevent user error or to insure the client doesn't end up bringing an inadequate boot or one that wasnt maintained. but none of the major guide companies require them for a summer trip up Rainier just to clarify.
if they are required for whatever reason or that is what you would prefer, i'd just rent unless you can find a dirt cheap pair or plan on winter climbing, trips to AK or other, higher elevs. just my 2 cents.
either way, have a fun trip and enjoy.