It’s always better to have all the track-outs. I do tons of mixing and I do get mixes from time to time where it’s vocals over a two-track and I generally do those really cheap for my clients. I can pretty much ALWAYS beat the two-track if I have the track-outs. That said, I get it, sometimes you are doing a mixtape or a promo thing and you don’t want to shell out the money for a legit beat. You just want to buy a two-track. While mixing vocals to a two-track will generally never sound as good as mixing vocals to the track-outs, have a pro do it will still sound way better than an amateur doing the mix themselves. Mixing to a two-track, particularly one that’s been compressed and mastered, presents its own set of unique issues for which there are lots of (often difficult) tricks we employ to make it NOT sound like vocals on a two-track.
So bottom line – if you care about the quality, hire the best mixer you can afford. If you don’t care, then just do it yourself.