Yeah, I was sort of in the same situation. Every project, I was buying needles, and a few sizes so that I could check the gauge. While I didn't mind building a collection that I thought I might eventually use, there are a few differences in DPNs vs straights vs circulars (and their various lengths), so I may have ended up with a lot of needles I'd never use again. So I just bought a set of interchangeable circular needles to hopefully avoid that. I haven't used them yet, but I bought the Denise ones, mostly because they seemed reasonable, with reasonable prices and shipping terms. And, they don't unscrew, they twist-lock. For my last project, I used long straight needles, and I found it awkward having the ends sticking off everywhere. Anything that can be done flat can be done on circular needles. However, and this points to your first question, my last project called for a circular and I used straight, only to find out a little later, why: It was a 2-colour project, and it had only one row of a new colour before going back to the first colour, meaning after that row, your tail yarn of your original colour was left at the other end. On a circular, you just slide the work to the other tip, but with the straights, I had to slip all the stitches point to point to the other needle.
After getting the interchangeables and looking at them, I can't wait to use them. You can even make 2 straight needles with them if you want to, and you can make circulars of all different lengths, even by joining cords together. Seems to be the only downside is you can't do small diameter circular knitting like on DPNs, you would have to use another method of circular knitting, like regular circular needle knitting, or magic loop, or some of the others shown in the videos here.