1.5 supports several whole new Arduino products even
Is 1.0.x obsolete, or does it still have useful features on its side?
So while 1.5.X works very well (it's "stable" in that it doesn't crash or work improperly), many pretty important aspects of the design are still likely to change. In terms of the APIs used by boards and libraries, things are still developing. I believe the Arduino Team is actually making a pretty good balance between making the new stuff available versus providing a stable platform that works with a huge number of boards, libraries, and sketches found across the playground and thousands of websites.