It does take up an invisible debuff slot as you say, but it’s pretty rare that it pushes off anything important. It’s one of the stronger DoTs if you have room for one.
While yes, the absolute ret average is fairly low, an upper quartile paladin competes pretty favorably with the averages of most other classes. A good ret isn’t holding your raid back unless your entire raid is high performing.
In a casual guild, a good ret is fine, and not competing with many players that will be blowing them out of the water. You probably won’t find many strict meta speed run guilds tolerating the spec though.
Assuming we’re using consecrate in a casual or try hard raid then you pretty much only have 3 necessary debuff slots and those are SA, FF and CoR. Everything else is inconsequential unless your tanks aren’t well geared and you absolutely need those demo shouts and curses of weakness.
Yes, Ret is the lowest dps spec when looking at absolute numbers and averages, and are definitely far behind rogues/warriors/mages (just about everyone is).
It’s not so far behind the rest of the dps options, many of which are still brought to raids, to matter. Rets will sometimes beat the rest of the pack on fights, and it brings more than enough dps to clear anything in the game just fine.
Sure, you don’t bring them when min/maxing for the fastest speed clear possible, but there’s also no sense in perpetuating the idea that they are bringing down your raid by existing.
Like in every other case, the player is more important than the spec they choose to play. Any of the specs can be a valuable addition to a raid team. A guy thats a terrible ret paladin will probably be a terrible rogue/warrior, and you’d still be better off taking someone else.
Pretty standard debuff list, it has only 1 real flex spot, sometimes two if the boss is not one with a taunt mechanic involved. Maybe you give it to a paladin to consecrate, maybe you do something else with it. Just saying, those 16 slots disappear faster than you think if you aren’t paying attention to what your raid is doing.