I can't comment about artists' acrylics because I have no experience of them, but regarding sticking to one paint manufacturer say, I would think most modellers with a bit of experience could use just one manufacturer's paints that suits them and their style etc., and create all the shades necessary by mixing colours. For example, I really like Revell Aqua Color, and I think that I could probably do ALL my models using these paints, whether hand-painting or airbrushing. Thing is though, once one really gets into model making, one wants to try different products, it's just a fact of (modelling)life really. I understand about budget, of course, but I doubt any model maker *just* sticks to their favoured brand, they just can't help themselves trying different stuff, and in fact, that's all part of the magic of this hobby :-)
Not sure if I've helped or not, I doubt it, but there you go.
cheers
Mark