Hivernica pretty much pointed out most of the points, I'll just specifically repeat this one:

for the love of god, please do NOT macro your fairy abilities with ANYTHING. Be it rouse + Whispering dawn, Physick with Embrace. Do not macro ANYTHING.

You'd want full control over your fairy (and your own abilities). You may want to pre-rouse the fairy and use whispering Dawn later. Or just Rouse in general or just Whispering Dawn in some cases. Sometimes you want to cast Physick on the tank and have Embrace on a DPS that got nicked by a little bit of damage. The Embrace CD timer is also different than your own and the fairy sometimes casts Embrace randomly at someone who got damaged. This can be a real annoyance if you're expecting to heal for 700 potency worth and the fairy isn't casting at all or is late.