To me, the rules are very simple; The overall band volume should be set to the quietest acoustic instrument, & mixed from that reference point. If the band needs a louder or more full sound, then the band needs to consider upgrading the PA to allow for reinforcement of acoustic instruments. Using a separate PA for just the drums is adding another uncontrolled element to the out front mix. For the same investment as buying a separate PA for the drums, you can significantly increase the headroom of the band PA, & that offers much more bang for your buck. If the band PA currently consists of a couple of powered tops on poles, & there's enough inputs on the desk, then adding a pair of subs c/w built in processing/crossover will often be enough to get the job done at lower volumes.
I've no idea what level of sound quality you're after/budget, or the gear your band currently has, but another route is to replace the band's existing tops with a full range powered speaker enclosure. We use 4 x QSC KW153's as tops with a bank of 18" touring pro grade powered subs, but a pair of those QSC's on their own are capable of handling a kit at reasonable volume. They're 1k RMS each, and have a very cool DSP section that means the 15" driver can deliver surprisingly decent sub frequencies. These will also give the band a great sound too.