The way it'd inevitably be presented is as 'self-improvement so that I may better aid ARM in the times to come', if asked for a reason.
And it's true; ARM's goals coincide with Xan's in that he really doesn't want to die and ARM represents his best chance of continued existence. So he'll use his super medical skills to better help people.
Didn't piecewise say that you can't get better than +4? That getting a +5 would only be good in the sense that it would provide a buffer for stat loss and let you use things like the Limiter Unlock more often? So unless piecewise told you that you need to get a +5 or greater, if you really need to succeed in a roll, wouldn't it be better to get a battlestim or a limiter unlock? Because unless said roll has to happen over a very long period of time or unless piecewise said you need to have Med>=+5, I don't see why you wouldn't get something like a battlestim or a limiter unlock.
He's said IIRC that levels past +4 basically grant further narrower margins of failure as tasks get increasingly difficult - so the common utility of a +5 Con skill, say, wouldn't be that different from a +4, because both could make any reasonable shot you bother to ask of them. The difference is the +5 could shoot someone's left testicle off from 10 kilometres away with a better chance of success than the +4.
And I'll be using battlestims too.