It occurs to me that the Citadel DLC has no small number of plot holes and irregularities (like the ending), but we don't have thread after thread dissecting those (unlike the ending). For instance, are we really supposed to buy that Wrex is already asking the Council for expansion planets? That Shepard would be okay with some R&R when the Reapers are advancing across the galaxy? That Tali and/or Liara are able to party when their planets are in peril (depending on when you throw the party)? That Shepard would play fetch with Eezo the varren with a frying pan? Or that varren can have biotic powers? Or that neither Shepard nor Joker would ask Brooks for identification when she seems like a mess and appears thirty seconds after they realize somebody faked their emails to get them to the sushi joint?
Haha, you have got some points here! Especially with neither Joker nor Shepard getting it they were both lured there by someone, and suddenly someone approaches them with an emergency. But... she fooled me too! I found it funny, because as soon as the action starts, I actually forgot that they both were lured there, weird how this sometimes works. And Brooks was really playing her role well. I'm sure there were some suspicious people, but I never was, she really got me, that b*tch 
As for the R&R and everyone having a good time... I had to find a way justifying it for my last Shepard because she was the mission first type, but then I kept that DLC until before the Cerberus HQs and said to myself it can only be a positive thing if the Normandy is in a top shape, as well as the crew, because after Cronos, there's no way back. So they can let go of a bit of emotional stuff, lose their sh*t and all. Sometimes, things like these help you cope with the bad things in life, you even downright forget about it because you're distracted by some good things. Escaping reality, I'm sure I'm not the only one knowing it 
The Citadel DLC doesn't really "fit" plotwise into the game, but nobody complains about it... because it's so doggone satisfying! Whether it's Uncle Urdnot bringing presents for the princesses, or the crew lined up at the top of the ladder shooting 48 times when one will do (Wrex's line), or EDI's eyes going crossed when she's your squadmate in the shuttle on the way to the Normandy, or Shepard's repeated "I should go. *I* should go. I should go" lines... the Citadel works. For almost every player, it gives emotional satisfaction and even downright joy, and for the most part, we overlook all the plot holes because of how darn good the Citadel is.
This is likely true, and it just really shows a bit more that people look for a more positive emotion I'd guess. I've also heard countless times that while the DLC can't be taken serious (which is true, because as you say, it absolutely doesn't fit with the plot and is just blatant fan service), it's fun. The question is if people would've really liked something similar for their end, but could be 
The ending? Not so much.
I'm not trying to complain. I'm trying to support Iedra's claim that a writer ought to try to give an "okay" ending or component to his or her stories; even if there are plot holes (really? A toothbrush somehow picking a lock? And when did all those crates and other cover get set up in the CIC?), they can be forgiven and overlooked easily if they produce emotional or cathartic resolution. And the Citadel, by and large, does.
It is interesting that even for me, as someone who liked the ending a lot, Citadel DLC was truly getting me back to happiness (ok, its ending not so much, but overall it's just a lot of fun). I only played it after seeing the original ending, and it was exactly what I needed 
I guess it's just really like you say, if you have a great time with your crew, then this likely just works for most.
I wonder now if things would've turned out different would Shepard not have gone alone up there and had some support, like you mentioned before. Guess we'll never know, but I'm sure squadmates' different comments, some maybe rejecting the Catalyst right away, would've made people more satisfied. Maybe.
It could've still been the same outcome, but maybe with the squadmates around, it would've made the situation less crappy for some. Not that I complain, as I liked what we got, but you know... for some folks this might've worked, too. I think it's especially true when we look at Dragon Age... Inquisition's ending was an "okay" one for me, but many people really liked it, because it was a positive ending. I think it's likely we'll get something like this with the next ME as well, especially after the whole backlash, so I'm prepared for a more "good" ending this time. Ideally we could get different outcomes, I'd really love that. But let's see. It will certainly be interesting how they tackle the ending this time 