I understand why you say that maxim but testing that this worked was the first thing I did and it does work. I was surprised as well. I haven't debugged all the way into the eval functions to see why this works but my guess is that the authors were pretty smart about figuring out which executable to invoke.
As for the problem I'm trying to solve, there isn't one exactly. This is more me hacking on something I don't fully understand because it regularly trips me up.
I do still wonder what would be the disadvantage of just configuring it to do --login by default and doing all configuration in profile scripts. It would be unconventional yes, but it would also make dynamic scoping of environment variable effectively opt-in via --noprofile rather than opt-out (which imo is how it should be). I would assume that uses extra resources or risks improperly handling crashed processes, but I can't find anything to that effect in the docs