I think soundtracks are more personal to me than most other albums because they usually become symbolic of a time in my life

Wow, I was just thinking about this very fact last night. I've only "discovered" a handful of artists via soundtracks, but when I have, it's been a nice little bonus. In college, I sold a lot of my soundtrack CD's back when I needed food/beer money, but the ones still remaining in my collection are:
Singles (probably the best true soundtrack ever)
The Crow (also good, nice mix of artists I previously liked and artists I got introduced to)
Higher Learning (bought it just for the two Tori Amos tunes)
Grease (my fraternity used Grease as a homecoming theme, so I had to buy it)

I hesitate to add Pink Floyd's "Obscured by Clouds" or "More" to the list because I don't think anyone on the planet has ever seen the movies they came from. "The Wall" isn't a conventional soundtrack either, but it did make quite an enjoyable movie.