The fundamental problem is that you can't do it without adding at least 16 bytes, probably 20, to the size of an index tuple header. That would double the physical size of an index on a simple column (eg an integer or timestamp). The extra I/O costs and extra maintenance costs are unattractive to say the least. And it takes away some of the justification for the whole thing, which is that reading an index is much cheaper than reading the main table. That's only true if the index is much smaller than the main table ...
Well, the trick would be to have it specified per-index, then it's up to the user whether it's faster or not...
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