The IntervalTree only maps partition keys. Since a majority of users deploy a hashed partitioner the work is mostly wasted, since they will be evenly distributed across the full token range owned by the node - and in some cases it is a significant amount of work. We can perform a corroboration against the file bounds if we get a BF match as a sanity check if we like, but performing an IntervalTree search is significantly more expensive (esp. once murmur hash calculation memoization goes mainstream).
In LCS, the keys are bounded, to it might appear that it would help, but in this scenario we only compact against like bounds, so again it is not helpful.
With a ByteOrderedPartitioner it could potentially be of use, but this is sufficiently rare to not optimise for IMO.