The problem with materia that grants abilities is that then some clases and jobs would be handcuffed to those and thus have one less option available to them. The same thing would happen if you did traits gained through materia (all melee would be forever shackled to an Auto-Haste, materia, for example).

I personally don't like the fact that the current system is driven by chance, not to mention that the stat stacking will come and bite us in the ass at some point.

That being said, I'm in favor of Airget's suggestion, as it favors variety to a degree. I'd reduce the number of melds per slot to 3 (or 2), and put the luck/chance factor directly on the crafting of the slots themselves. This is more because if we can easily get 5 slots per gear with a crafter buddy, materia is going to get nerfed across the board for balance purposes.