A counterspell can be a big advantage against healers. It will lock out their casting tree for 8 seconds while will allow you to blow a big chunk in their teammate. Counterspell mouseover is useful because it will not change your targets, but simply counterspell your mouseover target. If you dont have your mouse over a target, it will counterspell your current target.
#showtooltip Counterspell
/stopcasting
/cast [target=mouseover,exists,nohelp] Counterspell; Counterspell