Depends on whether or not you want it "meaningful" or not, because it's not just about whether or not you've known these people before.
Taking it at its simplest, there's two ways you can go through with either character, depending on your romance situation. Either you did, or you didn't (which can also apply for same sex scenario). If you did romance one of those characters, I would expect there to be more than just a casual reference to the relationship if I were to persistently interact with the character over the course of the game. It wouldn't just be that I knew Ashley in the past, but that there was a developed chemistry and history between them. A lot of dialogue could easily be shared, but if it's just "hey, you've known me in the past and we were close" at the start, and then the exact same Ashley regardless of what the player chose in ME1, then we really do not have a situation of any meaningful consequence coming from the first game.
For there to be any sort of plausible progression, I would expect that there would be a lot of very different lines of dialogue throughout the course of the game to reflect upon it. Otherwise it's as much of a gimmick as people claim it is now.
Combine this with the fact that, compared to new NPCs (such as Thane, as you mentioned him), Ashley/Kaiden will only be present not just those that have played Mass Effect, but still have their saved game to import into the new one, you create content could be impossible to be seen by a large part of your audience. Does it make more sense to invest time and effort into a character (optional or not) that anyone that picks up the game can experience, or one that can only properly appeal to those that played the previous game?
As you say, they hope to have content only available to those that have ME1, but they're still going to do it in moderation.