People actually like Cullen? I thought he was a wobbling coward in DAO that was more than happy to sacrifice innocents because of a fear they "might" become abominations. He was slightly less-annoying in DA2 as he wasn't as one-dimensional, yet he still continued to follow Knight-Commander Meredith almost until the very end. This was despite her draconian methods and harsh treatment of mages. Perhaps some gamers like him for his looks, but beyond physical appearance, he lacks substance and integrity. He will merely be a tool and nothing more in my Inquisition.
Whereas, in contrast, Leliana was actually an extremely likeable character. It was a mistake for BioWare to give the player the option to kill her in DAO. Now the community (five vocal fans who killed her) will never let it down and will continuously complain about how their "choices don't matter." The simple truth is BioWare wanted to further develop her character (should have seen this coming with her appearance in DA2 honestly) and decided a pivotal role in DAI was appropriate. I'm personally glad to see she is back as she was one of the best characters in DAO, and she has changed a lot since we last saw her.
If people really despise Leliana's existence that much, just don't acknowledge her or use her talents in your Inquisition. Problem solved.
Indeed - you are correct about Cullen 
But why is Leliana any better?
Come on, she is a zealot (which in itself is quite bad, as she follows the chantry without questioning it (she might not like doing certain things for it, but she does them anyway)) and she is dependent on someone holding her leash (firstly the Orlesian Lady who got her into to learn the bardic arts (poetry, singing, instruments - also espionage, poisons, fighting and killing), then Marjolaine (note: Leliana didn't as far as we know ever work independent as a bard!), then the revered-mother Dorothea (who became Divine Justinia later on), then the Warden and back to the Divine...and now? - Good question, maybe Cassy at first and the Inquisitor later on (when he/she has settled into the leadership-position of the inquisition))
In her way, Leliana is as one-dimensional as Cullen...as I already stated:
At least with her you can be sure she knows her craft - with him, I am not so sure (as I said, he is no General, he is the equivalent to a mid-ranked officer in todays military - something like say a Captain or a Major...no, not even a colonel (as that would be the Knight-Commanders rank - while the leader of all Templars and his deputies would be Generals))...still, I will not ignore either one on purpose (Gimping my inquisition is not a good idea IMHO - just because I do not like my staff...I can work with them, if I have to (maybe I get to like them, too - there might be some character development 
greetings LAX