For everything that needs to be converted, there is an extra cost attached - either in time or money, even both. Consider this for a minute. Supporting hex and decimal numeric entities requires a tiny module in the parser, and no added work for the poor schmuck who has to actually convert the documents. OTOH, choosing between hex and decimal saves only a few bytes of code in the parser while piling more work on said schmuck.
Support is trivial, nonsupport is extremely nontrivial - so go with support.
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