To be fair, you can add any external audio card to nMP (or to any Mac for that matter). I don't think that - regarding audio cards - the lack of internal upgrades is a real issue. On the other hand, though, I don't see what makes nMP so great for Logic Pro. Well, Logic Pro does run only on Mac, of course, but nMP has no h/w that favors specifically this application any more than any other modern Mac out there. FCPX sure, I hear that the dual GPUs come in handy. But for Logic Pro, I don't really see the true benefit. As a matter of fact, the enforced dual-only gpu (and the relative cost it brings) renders the nMP a non-ideal machine for a musician.
All the arguments regarding the old version of some h/w parts are valid, of course (like tb2 etc). But, let's be honest, the lack of h/w options and upgradeability is an entire Mac product line drawback rather than nMP's.