damn straight
if you want a lego kit, i'm all about that. the AR lower/stock is versatile and common, plus it's the GUN part, so makers can throw any damn thing they can on top of that.
spend $$$ on a good trigger, and in THEORY, you can move that lower around, though in practice, not always as nicely as you'd like; but if one gun is pure gas and another relies on buffers, or both? perhaps you need custom buffers to remember to swap out, or that bolt group doesn't play nice with Timney 2.1 lb pull?
i like buying completes. i even have one based with an AR lower, with custom springs/buffer - but i could not use that on another upper without spare parts/swapping around. for me, it's not worth it. all or nothing
that said, i do like that you can buy a complete that is jam packed with everything, and THEN you can tinker a little