Here are a few reasons to be PRO-66.
REASON 1
With 66 blocks, you can wet your index finger on each hand, touch your probe up to your finger on one hand, and run your other finger down a block and fine tone.
BIX blocks (I've never used one, but from what I've seen and read) are just like 110 -- hidden in plastic, you can't even touch the contacts. That would take a bit longer to find tone, secondly -- you can't short a pair as easily with BIX, shorting a pair with 66 to check to make sure that the tone you detect is the correct one is as easy as placing your probe's tip across the pair.
REASON 2
66 blocks are EVERYWHERE. Sure, we could start a trend and replace them with BIX blocks -- but they're never going away (66). Why reinvent the wheel when the benefit is minimal?
REASON 3
I haven't seen that many 66 blocks fail, crumble, or whatever -- and if they are, maybe your wiring closet is a bit too exposed to the elements.
REASON 4
Replacing or putting in a new 66 block costs nearly nothing, they're cheap as all hell. Bix blocks are not. 110 blocks aren't that bad, but they aren't anywhere near as quick as a 66 block.
REASON 5
A 66 punchdown blade is a whole lot less mechanical than a BIX blade. And cheaper too...
Bottom line? BIX is more expensive, doesn't have as many uses, and wouldn't be beneficial enough for me to justify replacing other blocks, or using them for installs.