Hash tables aren't functionally equivalent even if you use numeric keys. Deletion may be equivalent as a byproduct of mIRC's implementation (I haven't checked) but insertion certainly won't be possible without manually shifting all higher indexes - now that definitely would result in unacceptable performance.

The hash table API also isn't very descriptive when applied to arrays - for example would /hadd insert a new item at a given index or overwrite it? Adding support for slicing in relevant commands would add an unnecessary layer of complexity to the /h* commands making them quite different depending on what kind of 'hash table' was being used, not to mention there would also (presumably) be additional commands for arrays that would have no relevance to hash tables (eg. sorting). All in all having two different concepts sharing a set of commands would be extremely messy and would only confound most scripters.

Spelling mistakes, grammatical errors, and stupid comments are intentional.