Jumpsuits PRO and CON:
PRO: If you envision a situation where you'd need to QUICKLY(i.e. under 3 minutes) don something "tactical" over civvies/mufti, jumpsuits work fine. John Plaster recommends them in his "Ultimate Sniper" book/video for that role. If Nomex's fire-resistant qualities are not an absolute necessity, surplus Army OD green mechanic's coveralls would be pretty serviceable, or even brown/khaki Carhart's/Dickies coveralls should suffice.
CON: In a warm-weather situation, coveralls get HOT, and you can't take off the top as you would with BDU's. Yeah, you could shrug out of the top portion and tie the sleeves off around your waist, but that's clumsy and looks like crap.
CON: When it comes time to do what bears do in the woods, jumpsuits are REALLY clumsy and slow. With BDU's, you simply drop trou and do your business - with coveralls, you've gotta take your pack, LBE, vest, etc. off, strip down, and then put everything back on again. I do know, however, that US Army Nomex Combat Vehicle Crewman's coveralls have a "Doctor Denton" flap for that purpose - I don't think that Army mechanic's coveralls or Nomex aviator's jumpsuits do, but I could be wrong.