Well, from what i have spent in pheons (zero), i don't think it's a really successful strategy. I have 500+ of them, bought some pieces and be done with it. The fact you don't have to constantly upgrade stuff means you have literally to buy things once.
EDIT: my points is that yes, liketly they're made to make people spend more money but they are not actually needed overall, the free ones you get are enough to buy anything.