I'm not sure how you expect it to work, but I use it in my Cleanse macro and it seems to work just fine. E.g:

/stopcasting
/cast @mouseoverui MyCleansingSpell

This will abort the current spell (if I'm currently casting a spell with a casting bar) and instead throw a cleanse on whatever character I'm hovering the mouse over in the raid ui. It's a single button click, no need to click anything else.