Come on. If it's "purism" that you're after, then I'm sorry but you have to build your own instruments from scratch
Timbre is just as important as harmony and rhythm. So if you're using a Fender Telecaster to play a guitar part, then you're basically using a "prepackaged" timbre (including the amp, stomp boxes etc) which you didn't create yourself. Then if you go ahead and lay down a synth part with a preset, you're cheating again. Surely you should program all of your sounds from scratch, on a synth that you made yourself.
Given that music consists of harmony, rhythm and timbre, I really don't see how using loops makes you any less of a musician than someone who uses presets, or dials in a guitar sound that's based on 1000 guitar sounds he's heard before, or plays a riff that's hugely influenced by riffs he's heard before.
Of course there is the extreme case whereupon a musician takes a loop, loops it, and that's the song. Is he any less of a musician than someone who plays an open D chord on the guitar over and over again in a straight 4/4 for 3 minutes? OK, so neither one has really composed anything. But notice that in the latter case, playing his own instrument does not necessarily make him more of a musician. He didn't "write" that D chord, nor the rhythm.
So imagine Loop Guy finds another loop that goes with the first, only after some experimentation he shortens it by a beat in order to get some kind of polyrhythm going, and transposes it into a different key to create some harmonic curiosity. Meanwhile, Guitar Guy throws a Gm chord into the mix. Has either overtaken the other in terms of musicianship? Does Guitar Guy's boring D/D/D/Gm "loop" make him any more of a musician, just because he came up with it and played it himself?
My point is that no matter how you approach the task of making music - whether you use loops or you compose everything from scratch and play it yourself - the level of creativity and invention that's involved is up to you, and is what rates you as a musician. It's perfectly possible to create pieces comprised of loops which display a high level of musicianship, just as it's possible to create pieces comprised of original playing which display a low level of musicianship.