1 immo = the cost of 1 pheonix. If you don't have immo and only 6 pillers/pends to depend on then it is a dead card for a while.
1 immo = the cost of all 3 minor pheonix.
1-2 pillers = 1 Minor pheonix in 1-2 turns.
Minor pheonix can be summoned within 2 turns of drawing it 90%+ of the time. Pheonix is no where near that. Also if I draw 2 Pheonix then I need 2 immos to summon both.
Minor Pheonix makes this deck LESS combo based and thus harder to counter.
Also remember we only have 3 main targets for immo anyways, to depend on immo alone to get one of your main cards out if foolish.
I play tested with MP's and they are better. I know I can get them out, pheonix is too much of a gamble. Also remember that lava golems are in this deck, playing MP allows for them to be summoned with immo too.