Yak shaving is only procrastinating if it's optional. Any interesting thing you create will need a huge set of errands into unrelated tasks. If you refuse to do the errands, you'll never do anything interesting.
If the series isn't finished because of TEX, I think we'll feel like it was a bad tradeoff. (But I could see others feeling like TEX is is more valuable than volume 11 or whatever of TAOCP).
If working on it is procrastination or actually a better use of your time than solving the original problem is never obvious.
Anyway, I'm on the side that thinks TEX is more valuable than TAOCP.
1. Any apparently useless activity which, by allowing you to overcome intermediate difficulties, allows you to solve a larger problem. I was doing a bit of yak shaving this morning, and it looks like it might have paid off.
2. A less useful activity done consciously or subconsciously to procrastinate about a larger but more useful task.
I had thought that only definition 1 counted as Yak Shaving but apparently definition 2 is also in common usage.
I would personally find the concept more useful if it only meant definition 1. But that might just be me.