In version 10, Mathematica not only added
Activate but also highlighted this change in the "New in 10" page (in Core Language Enhancements). Thus, I suppose it should be something useful. However, I didn't understand in which sense it is better than the good old
ReleaseHold mechanism. I noticed a few differences, but all of those are minor:
Inactive objects are printed in a lighter color.
HoldForm objects are printed in normal color.
(2) There is an
IgnoringInactive function to include inactive objects into pattern matching.
It would be nice to hear if there are more important differences.