Are you suggesting creatures come out of spawn points at any time or would the standard spawn point rules apply and it would be only during deployment phase but they just aren't inactive (summon sick) their 1st turn?
I like the idea overall, especially the suprise/support element. My general view is pretty against changing something that is already in print (see my comments on not nerfing/changing Temple of Light etc).
BUT that isn't to say there isn't ways (even FUN NEW ways) to make that happen. I think it was suggested somewhere to have a "face down trap card" that is ambush and it lets you summon creature(s) out of it as a "surprise" element. So you could put a face down zone enchantment that lets you summon a creature when you reveal the enchantment. Since you can reveal an enchantment almost anytime it would sort of do what you are talking about BUT it wouldn't be so overpowered. Maybe it is a one time use sort of thing, plus it is limited to the zone the enchant is in (but so is the spawnpoint).
Personally for me I don't look to my spawnpoint as a return on my mana investment, the point of the spawn point (for me) is the extra action it allows me (THAT is huge). I've even played as a Beastmaster and I went creature heavy but I never summoned a single animal the entire game (I let the spawn point summon them all for me) and I still won with little trouble because I use my FULL ACTION to do damage rather than "wasting" my action phase summoning a creature, I nearly always summon creatures during deploy phase not action phase.
Anyways I think the idea sounds AWESOME, but I'd love to see it implemented in a way that doesn't change existing spawnpoints. So either a new type of spawnpoint or the enchantment/ambush type thing would work and still provide balance so it isn't overpowered.