I don't really see how a mystic adept taking the adept spell power would work fluffwise,
In case you're actually interested, here's what me and my GM have agreed on:
Firstly, there's nothing in the rules saying that adepts (non-mystic) can NOT follow the magician's way
Secondly, the way the mystic adept rules currently work is that they're ALWAYS "magicians" first and adepts second (while in SR4 the player could chose where to lay the priority by splitting their magic), therefore it's currently NOT possible to make a mystic-Adept that is an Adept first, but also dabbles in spellcasting. You HAVE to get as much Magic as you want powerpoints, and THEN spend karma on the PPs...
In our headfluff, 'Adept spell' circumvents that, albeit at a high cost... and discounting it via the magician's way makes sense in that regard. It should also be noted that my GM allowed me to take it multiple times (once per spell), although only the first instance may be discounted.
Which, in the long run, means that this character can now chose between a powerpoint, a spell, or a metamagic any time she gains an initiate grade... Quite pricey, those spells, no?