I guess I've pursued the romance with Ash because she seems the more "mature" of the two offered in ME1. I romanced Liara on a second playthrough and felt the relationship was kinda shallower, in spite of the all the mind communion Shep and Liara went through. Went back for a third playthrough and romanced Ashley again.
Sure, my Shepard was bummed out by Ashley's refusal to join the team in ME2, but she had plenty of good reasons not to:
1) Dereliction of duty - she couldn't go AWOL to follow Shepard, as Shepard's no longer Alliance military
2) She had to report to her superiors on what occurred on Horizon
3) As Chief of Operations on Horizon, she had her own people to care for (she didn't install those turrets by herself).
4) Cerberus did some really nasty stuff to Alliance soldiers in ME1. A lot of Sheps out there do not seem to care... so much for being brothers in arms! I don't blame Ash one minute for turning me down.
BUT...
Now Shepard's a SPECTRE again... he's got his own ship with a crew that has little or no loyalty left for Cerberus. Wherever Ashley's next shore leave happens to be, Shepard will be haulin' his "great ass" over there (after calling his mother, of course...

).