As you state, the 6D doesn't even have that. Just sayin'.As you're the expert on Canon gear: Are there any redeeming reasons for Canon implementing the 5d2 af in the first place and then copying it to the 6d (+2 non-cross points, +low light af)? Maybe designing an af array for ff is more complicated/expensive - even though the af spread on 5d2/6d isn't very large?
Well, first off there was no 'implemented AF' in the 5DII - Canon simply reused the AF sensor from the original 5D. Many people compare it to the 20D/30D AF, but of course the sizes are different so the design is different. The theory of the assist points was nice - 6 'invisible' points to help with servo tracking. But in practice, it didn't work well, I suspect in part because of a slow AF chip and weak algorithms - and I bet they reused the whole system (AF chip and code) from the 5D in the 5DII, meaning a really old AF system. For sure it couldn't track well, and enabling the assist points (which are off by default) didn't seem to help. As for why, reusing the 5D's AF saved development and production costs. Similarly, when the 40D got a new AF array, the 20D/30D's system went to the Rebel/xxxD line, and eventually the 40D/50D/60D's AF went to the T4i/650D (which I expect means we'll see something new in the 70D, my money is on 15 points, 9 crosses </speculation>).
The 6D got a similar system, looks like a modification of the basic 5D design. No more assist points (too bad, IMO, because with a modern AF chip and current algorithms, those tightly-clustered points may have been effective. I'm not sure I buy the bit about needing 'elements of the AF system to be larger' to support -3 EV. AF point size and threshold determine micro lens size, low light is more about signal amplification, and given the density of the 5DIII/1D X AF sensor at -2 EV compared to other bodies at -0.5 EV, I don't see it.
Why 11 points on the 6D? I'd say pure marketing. The spread is no greater than the 5DII, just two extra points stuck in there. But consider the 6D's market position - 'entry-level FF'. Targeted to Rebel/xxxD and xxD users looking to 'go FF'. So, the target upgrade audience all have bodies with 9 AF points (and 18 MP sensors). So, give them 11 AF points (and 20 MP). Canon's marketing department understands that for most consumers, quantity trumps quality - the former is a simple 'more is better' and can listed as a top-line spec and be printed on a display placard at Best Buy, the latter requires some technical understanding (e.g., why 9 cross-type points are better than 1 cross type plus 10 lines). </cynicism>