I like aforementioned acoustic songs because the are so simple. I mean, sometimes I find myself skipping some of the big, epic songs to have a good listen to the acoustic ones, in fact I do it a lot. The songs are a nice break from the bigger ones, and they may be simplistic but they are not boring, they are nice and relaxing and emotional. I mean, Opeth is about more than simple chord structures and all that, but I think the simple acoustic songs add some welcomed variety. I don't see exactly what is wrong with a break in the action to relax to a nice, simple, emotional and to the point acoustic song.
On the other hand, on Damnation, which will consist entirely of acoustic songs, they will HAVE to do something rather new and interesting to make the disc good. I adore credence, harvest, benighted and those, but a whole disc of songs like that would be kind of boring. I don't think they need to make the acoustic songs on the other CDs innovative and amazing, but for Damnation they have to or it will be a bit lame.