I know a lot of guitarists like to use delay to emulate reverb; but for me, it doesn't do the job as well as a dedicated reverb. I can't fine tune the parameters as thoroughly, with a delay, as I can, with a reverb.
I sometimes use both, combined, but I have to be careful about overwhelming the mix, or I lose all note definition. That's been a hard lesson to learn (and I'm still learning it LOL).
All of my boards have at least one reverb and one delay. Most of my boards have at least two delays on them, and they usually have more delays than reverbs.
I use delays mostly as an effect, for creating rhythmic patterns - whether it be cascading, syncopated, etc. (with sometimes, more than one delay, combined), whereas reverb is a space emulator, for my needs. A delay will never give me a big ambient wash* (if that's what I'm looking for). So, in my mind, both have their purpose - sometimes combined; sometimes not.
* There are exceptions to this. My DIG or my Halo (both, dual delays), can create a very beautiful reverb-ish sound that I find very pleasing - but I still prefer having at least one dedicated reverb pedal available, at any given time.
I don't think I could bring myself to ever choose one over another. For me that's like choosing salt over pepper, or soup over salad. If given the choice, I just may want both - and I have plenty of them at my disposal, so why not use them :-).
Depends on the need for the song.
When I think of delay as a reverb, I envision a pipe wrench, rather than a hammer, for driving a nail. Both will work, but the hammer is more effective and efficient a tool for the job - at least for my uses.
Obviously, opinions vary.
No offense meant towards those who are fine with using a delay as a reverb. There are no wrong answers. People like what they like and what their pocketbooks and available board space will allow.