Our pool is 3'9" to 9' deep. The overall length is 54' x 23', but 9' of the length is a 9" deep sunshelf. We do have a main drain, and I leave that valve most of the way open so some water is being exchanged from the deep end if that makes any difference. Our heat pump was installed with the pool, so I can't comment on extra costs. We did have one part go on our heat pump during the fourth swim season, but parts and labor to have someone fix that were only around $200 total so I think it's holding up fairly well.
Thank you for the feedback. The pool has three skimmers, which keep it completely clear of surface debris with however far open I have the main drain valve. I run the pump about 18 hours/day mostly at a low speed that keeps the SWCG happy but for some hours at a higher speed for skimming. I might play around closing the main drain valve a little bit more next season to see what happens, but all is working well with it somewhere over half open. I understand main drains aren't strictly necessary. But they come in handy sometimes! e.g. I was having some plaster repair done in the Spring last year that required draining below the skimmers, and I was able to run the pool with just the main drain, which was helpful since the company took FOREVER to complete the work. Partial drains are also easy with a main drain, although I also have a pump for winter when the pool is closed. Personal preference, but I would put a main drain in any future pool I built (which is hopefully something I will never have to do given my first nightmare experience with that process!)