I usually need to test transitional animations between animations and this really helps to see what they look like.
I can't think of a case for when you'd need to not reset the time when you switch to a different animation, but like that other thing I thought was not sensible, who knows about the weird (but hackishly efficient) ways people use Spine. :p
It's fortunate that it's not that hard to set up a scene to test animation transitions in Unity. It usually ends up as a more faithful animation test compared to the rough previewing this change of default would provide. But definitely I think having it reset (by default and/or as a checkbox thing) is something very useful for just animators who have nothing but Spine to test their animations in sequence.
Or Spine could have a more explicit thing for testing and playing Animations in sequence, but I think that would be a million times more complicated to implement compared to this change. And I think reset-to-time-zero would be the sensible default either way.