IMO, there is almost no reason for you not to click it every time it pops.. now if I'm canni'ing I heal myself in between spell casts.. or if I see an off tank going down fast, I heal with my regular heal, click it, then do my regular heal again.. that's like +900 to each heal if you want to think of it that way..
anything that lets you cast spells outside what was designed = powerful affect. It's not about no mana or a small heal, it's about doing something when you normally just sit on your ass.. that's also why I still use a mod rod =b