First off, don't turn me into a strawman, that's not what I'm on about. I thought he had all stats to max/have it distributed evenly that he has no sp to allocate.
Second, this tactic you're saying is terrible. It's counting all future foes to be slower than 100 agi. This just seem even more convenient for the mc and it makes him very boring in fights.
As far as we know, nothing previously encountered couldn't be beaten with stats half that - case in point, Team Simon has stats around 50 at most, and TS completely defeated Evans dungeon at that stat level.
It is good strategy (in military terms) to allocate a reasonable force to each point that exceeds an expected attack, then hold a central reserve to reenforce where needed once an attack begins. And that is in a situation where you CAN deallocate those reserve troops if it turns out to be a feint.
Its also not impossible that he could have beaten Enkai at 100/flat - he could barely track Enkai's attacks though, so he was probably a bit lacking. Enkai is OP by any reasonable measure for the 18th floor, so is a black swan event.
20/20 hindsight could suggest that had Kei allocated 200/flat, that would have been good enough for Enkai, but he had no way to know that until he encountered Enkai - and had Enkai needed unbalanced stats in a different pattern, that might have left him lacking enough free SP to boost those stats at need. The benefit of 20/20 hindsight is that the actions that seem "reasonable" in retrospect were just one of a thousand options without that benefit.
So in short - there was no reason to assume (in advance) he would need more than 100/flat, so he prioritized flexibility and his humanity (as he struggled to get used to running at 100/flat) over using up all SP as fast as possible, and in the actual event, 100 was good enough for him to adjust to the needed stats AND still have a reserve in case he needed more. Allocating 100/flat didn't mean he couldn't go to 200/flat if he needed to - it just meant he could go to 300 or even 400 for selected stats if he needed to, which would be an option he would lose if he had gone to 200/flat in advance.