As a general rule I find fan service to be bad. It is often, but not universally, of dubious quality and of dubious merit. Cameo fan service is the worse. In some cases it can work but in most cases it doesn't. I really really hope not a single member of the Normandy crew every makes it to ME:A. Especially given that it is 100s of years post ME3, at least that is my understanding. I have no problem references to the heroes of the Normandy as that would be history, the lore of the setting. But saying 'hey lets have Liara make an appearance,' just so people can go tee tee tee and get all warm and fuzzy inside would irk me.
The game's content should service the narrative not fans.
Re: Citadel DLC
This was for me an example of crappy fan service and good fan service.
I did not find the party out of sorts but it did bother me that people working an the most important secret project in the galaxy took time out to travel all the way there for the party. Kasumi I am looking at you! why are you not working on the crucible? Frankly not every alive member of the series should have been available for the DLC. Content serving the narrative not fans thing.
The whole clone thing was a WTF, yet it could have been so much better written. Imagine they used clones in the Lazarus project to replace organs for Shepard. now imagine one of these surplus Shepard's wasn't disposed of and they came after you but they were obviously missing human tissue and synthetics were use in replace of there organ's/limbs. i could see that clone being very very bitter towards Shepard as they are literally not whole because the original was given their body parts. This seems a far more plausible far more interesting clone story which is much the same but not some stupid lets replace Shepard with myself.
The entire plot of the Citadel story content seemed like fan service so we can have running gags while we played it. It just rang hollow to me. but the idea of wanting your current crew and companions to blow off steam before you take on Cerberus and earth makes sense to me. If not trucking in every possible companion regardless of their current assignments fighting the reapers. There should be people who can't make it. it a fraking war for christ sake. How did everyone get long term or short term leave all at the same time over different branches of the service and different national services?