Okay, lets put this into perspective.... I expect better than you than to keep parroting back... one sentence cautioning GM's that the ability *MIGHT* be abused. Might is not WILL.
Even if they do spend 10BP... what do they get... they get a gimped mage if they don't split their powers. If they do go 5/0 split... there are a lot of ways to make them pay for this. Unless they pick some kind of dual natured or a completely munchkin pick like a pixie... they need to spend a full point of magic for astral perception. (yes, runners companion cheese is a bigger problem than mystic adept). Without astral perception... they can't clean up their signatures after them. Which gives you yet another tool to use against them as a GM. That's a big 2x4 to wield right there.
If you don't allow them to initiate and gain PP at the cheap initiation karma rates... they're going to have to pay full freight for assensing... which is a 10BP quality PLUS a 10BP rank in the magic... that's 20BP right there... just for a gimped mage which can't project. So much for a 'cheap' buy like you keep parroting back....
What do they gain... they gain the option of access to adept metamagics for one... they don't necessarily need to spend PP on adept powers... they can spend it on adept metamagics instead. That also counts as exploring both sides!
That's why I'm saying you're parroting BS Midas... the numbers don't pan out... unless YOU as GM are playing powder puff league and not playing out the consequences of a mage ignoring the astral. (lack of access to the astral is one thing... being completely unable to interact with it is quite another major drawback).