I think making Now API-interactable would just beg for spammy popups. Mobile is already a maelstrom of apps trying to promote themselves, or getting paid to promote others.
Google already has a system in place for managing apps and services' access to your account[1]. Not to mention if you install crappy apps you are already going to have issues[2]. People need to be careful of what they install and give access to their account, I can't see how a Google Now API would make this problem worse.
A Google Now API would be awesome.