Here's how I look at it:
Customizers like BrickForge, BrickArms, and MMCB are made to accommodate Lego products. Customizers can give us similar quality to TLG's pieces, but in different genres in which TLG hasn't gone or won't go.
Clone Brands are made to copy Lego's tube-stud connection. Clone brands can be cheaper, but have inferior quality, more prefabs, and models/characters/whatever-you-call-their-minifigs that don't work in the Lego minifigure scale, etc.
People who see it as one whole medium is cool for them, like Dave and John Xandegar of Brixwerx. I think that's great, and if it's for you, good for you. But people who are absolutely convinced MB is totally better than Lego, then they must be outta their minds.
As LoO mentioned, just the inferior quality turns me off since I already have Lego, and customizers give me things that I want for the scale (for example, I'm using BrickArms to create Team Fortress 2 characters).