Well, perhaps it a greatest hits collection? It is nice to have all the tracks in place. I actually don't like them on the comps, they sound too different from the other material on the comps.
There are just so many artists who are capable of creating an interesting 3-4 minutes of ideas, but then try and stretch those ideas out into long mixes which end up making you sick and tired of the ideas you liked. In other words, the material overstays its welcome.
My opinion is that the long ambient tracks are okay, but get old really fast. I think the gold standard is:
FSOL Lifeforms.
http://www.discogs.com/release/1130474
Lots of mini tracks on there.
I think ambient died in the 1990s because people got board with the long over indulgent tracks.
Can you give me an example of a long ambient piece that doesn't get old fast?