I noticed that a number of Be Quiet PSUs have a single 8 pins PCI-E power connector (that's rated for 150W).
However, most GPUs have one or two 6 pins connectors (rated for 75W each).
Does anyone know what the purpose of the single 8 pins connector is? 2x 6 seems much more logical to me.

http://www.bequiet.com/en/powersupply/241

It is actually an 8 pin cable configured as 6+2 pin.

True, so it's usable for 1x 6 and 1x 8, but not for 2x 6.

Yeah, and so far graphics card manufacturers have used 2x6 even when 1x8 would have been fine. Many such graphics cards come with a 4-pin molex to 6-pin adapter included in the box.

So nobody knows why they ship with 1x 8 (6+2) instead of 2x 6?

My guess? Segmenting. Low power units with one lead for single-GFX setups, then mid- and hi-power units with more leads for SLI and whatnot.

I actually got curious enough to check this, and sure enough the <400 W units are marketed for single GFX setups in every BeQuiet PSU series except the SFX and TFX series, obviously, and the >500 W units have more GFX power connectors. This was my impression from a cursory look through all available series at http://www.bequiet.com/.

Guess they decided not to take any chances.

Good point. 2x 6 would support 2 150W GPUs (connector-wise).
Too bad GPUs use 2x 6 instead of 1x 8.

To me it seems everyone forgot about the 8-pin PCI-E connector and just jump all the way up to 6+6 if their card even looks like it's going to be power-hungry. I haven't personally used anything but 6-pin cards up to this day.

I guess the main point is that it doesn't make any kind of sense that there's a 6 pin connector rated for 75 W and an 8 pin one rated for 150 W (the pins being the same)... There should really have been, say, a 4p connector (75 W) and an 8p one (150 W), with the ability to plug two 4p plugs into a 8p socket - just like for the CPU power connector. PSUs would then just provide as many 4p connectors as is deemed reasonable for their power rating... But instead, we got this mess.

Since the whole 6p/8p system doesn't make sense to begin with, I wouldn't hold my breath for the video cards and PSU manufacturers to finally get their act together any time soon...