I left Ashley to die about 95% of the time. She just wasn't that useful to me from a gameplay perspective. Tech abilities were extremely useful in the first game due to all of the Geth, with Biotics being excellent for crowd control. I used Kaidan and Garrus as my go to team, subbing in Wrex or Tali depending on whether I needed extra damage or hacking.
With an Engineer, Adept, Infiltrator, or Sentinel Shepard, the combo of Kaidan's tech and biotics and Garrus' combat prowess and tech meant I could lock down the entire battlefield in about a second and a half, letting Garrus assassinate whatever wasn't presently overheated, on fire, in the air, or held in stasis.
The power loadout in ME3 makes the Garrus & Kaidan combo even better. My fast casting (as opposed to hard tanking) sentinel can set up tech and biotic explosions off of them, and vice versa. With explosions and bursts going off left and right, I'm able to mow through enemies like I used to in the days of Chemical and Polonium ammunition.
I did a dramatic role play run through the series where my Shepard pursued a romance with Ashley only to have to choose the mission over her life, but it wasn't even worth it. Without a living love interest in your ME import, ME2 pretty much just assumes you came down with Blue fever and hooked up a power coupling with Liara.