I'm biased since she's my favorite character. I always bring her on board, and take her on most missions for the last half of the game.
I suppose if I had to look at it more objectively... She doesn't talk much at all on the Normandy and rarely leaves the observation room (I think she ventures out into the ship itself, like, one time at most). In terms of on board conversation, there's not much there, especially if you're not romancing her.
Survivor is right - her mission dialogue tends to be good, though. Her Citadel conversations are also decent, albeit very focused around her family.
I also think she's a great asset to have for the missions. I give her a Revenant and then just let her go wild. There have been a few times where she actually saved me a reload by stopping me from dying. Most memorable is the time she took out a geth hunter that got the drop on me and was about to finish me off.
At the end of the day, I've never done a trilogy playthrough where Kaidan survives past Virmire, so I can't speak to how the quality of his content, on or off the Normandy, in ME3 compares to the quality of the content that is there for Ash.
Since you're not as heavily invested in her as I am, I think what it should ultimately come down to is who you feel is better suited for your needs in terms of the mission. Do you play a Shepard who already has biotic abilities but maybe needs some extra firepower on his/her squad? Then go with Ash. Or do you play a straightforward soldier who could use someone with a biotic/tech skills combo? If so, it might be better to have Kaidan around.