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In Fortress, yes, it's definitely a useful skill! I used to love Halon (Tower) as my favorite hero – he was a specialist in mysticism. But in the regular Heroes of Might and Magic III, mysticism isn't the most useful skill – I agree!
I don't know, my favorite hero in the Tower is Solmyr (remember, his specialization is chain lightning). If you max out his air magic to expert level, it will be devastating against large groups of enemies!