I've always been fond of Dominions personally.
For new members looking to find themselves situated within any Major Faction, Dominions are probably one of the easiest ways to come in and meet the members of their respective/chosen Factions. This could be countered or argued of course by saying that it's a Faction Admin's responsibility to usher new members into the fold, yet there's something simple about jumping into a Dominion with a set few objectives and story already laid out to get involved in.
Completing dominions also give factions a sense of achievement, much in the same sense as Invasions yet without the stress. They tend to be easier going and providing the story setting is appealing to the members of the Faction involved, they can be finished rather quickly if people really fall in line with the narrative. Not only can Faction Narratives be carried out within these threads, but so too can personal arcs and character dev be found through Dominions.
Looking over the history of a Major Factions Doms can instil a sense of pride in the writers, making them feel like more of an influential part of their respective Factions, making an impact while carrying out their individual/faction stories.
I personally wouldn't want to see Dominions removed entirely. Yes, they can seem like a grind if a Major Faction repeatedly runs one dominion after another, speed doms or fails to provide a captivating story setting, but I don't believe these are issues with the system we have, as opposed to a lack of appeal in the content that's provided within individual dominions (those that have felt like a grind in the past).