As this comment metions, using macros in AoE2 is not really useful. Using a bot, however, is useful, but XMouse is not a bot. It cannot be used to select enemy troops or enemy buildings, and the only thing it could arguably be used for is go through each building of a type to build things (i.e. to queue troops, or queue farms), which does not give a significant advantage to players.

It can be used to sync / crash games.