Oh I get that, qazplm - not totally disagreeing with you. But Painter has said what his preference is, and in part it depends upon whether or not guys leave. I think one difference is with recruits, you generally have more time to develop the relationship and find out if it's the right fit (including right personality type), which Painter emphasizes.
It also takes time with Painter's system - they run a LOT of plays (or better put, have a lot of action tangents), and the defense is probably more complicated than others just because it relies on connectivity, and guys knowing it inside and out. Not to say that can't be learned quicker with some.....but it's perhaps a crash course for some. You also have to have trust from the players - both ways.
I also maybe am in the minority thinking Painter also prioritizes the chemistry with the current team.....so if he makes a move, he maybe is more risk averse toward disrupting the positive stuff he has going.....but absolutely if a guy is available that he knows the game and fit and thinks he would make the team better, he's shown he's not opposed to it.
Again - I agree it's a resource, but to me, it's not just a plug and play situation. Now, it worked fabulously with Lance Jones, no doubt.
JMHO