The flat 40 mana from Alira can be extremely helpful later on if you're having trouble sustaining your AA or spell casting. Because this build gets so much % inc. mana from the tree, that 40 flat mana turns into much much more. I personally recommend going for gear that has flat mana rolls on it, as it will be very very beneficial for you, however, don't rule out this reward as an option if you need it.