I just figured the off-by-one would be unnecessarily confusing to anyone trying to use this. But i don’t have a strong opinion, and we might favor the small confusion over restricting the coinbase’s nSequence.
The height could also be encoded in the coinbase’s nVersion, and if we think making the block height available in a simpler manner than parsing the scriptSig isn’t a goal then i think just requiring the coinbase transaction nVersion be anything but 1 would be sufficient.