The US experience with that showed that an overall organization was needed, or they wouldn't work together and coordinate well. It's part of why we no longer raise a unit and leave it at home. We raise a unit, station it, then rotate the personnel between units for turnover and to cross-pollinate. Everyone gets the same training and keep levelling out local ideas so all units are more or less on the same page. I'd argue that an army would prove necessary just so different worlds didn't have better or worse local defenders, and so that those troops would be loyal to the whole UFP, not the local gov't, and therefore less likely to seem a potential threat to other member worlds during tense political crises.
I understant that it wouldn't be a stratetigally best possible arrangement, but I still think it would be something Federation might do. I could easily believe Federation not expecting needing an army.
UFP is not an unified nation state but an alliance of different worlds. It's more like EU than USA. Member worlds might not like the idea of large numbers of UFP troops permanently stationed on their territory. And yes, having a permanent army would make Federation more militaristic.
Oh, and NATO and UN forces kinda operate the way I suggested, and they seem to work just fine. NATO has no army, UN has no army. Their member nations have armies that occassionally perform joint operations.