Shatner wore lace-up boxer boots for his location scenes in "Arena." I think this was because the episode before last was "Shore Leave," in which his big location fight with Finnegan destroyed a pair of the expensive hero boots. And the show wasn't about to do that again, not on their budget. [Edit: I didn't recall the lace-ups in "Tomorrow is Yesterday." I'll bet Greg has observed TOS more closely than anyone in the world.]
When I was a kid, TOS was my principal point of reference in life for what looked cool. Back then I would have given anything for a pair of Star Trek
boots, but not with the "Cuban" heel you see on early Spock and McCoy. That type of angled heel originated with cowboys who needed to keep their feet in the stirrups.
Today I'm over it and would not consider laying out $800 for a pair of boots that I'd have to baby and never scuff. The grownup Zap wants to pay less for footwear that babies him and can go through snow and rain.