Also because doing the same thing over and over again "[because X for X in (consistency, security, esthetics, whatchamacallit)]" grows old fast - and by the time it's been fixed and running again, X is no longer there, or it's been totallty twisted out of the intended shape.
I think the latter is a much lesser risk, assuming the "works" in "if it works" is indeed correct. After all, sure, /usr has a funny history, but in practice it doesn't really bother anyone.
That could be the case. One example of the cost of stasis: the / vs /usr requirement in the FHS might stop or complicate future technical possibilities that use the filesystem (packaging systems, distros, copy on write snapshots etc) from being possible. Sure it's a low cost in this instance, but there's always a cost.
