The choices were 7,14,21. The point being, in answer to no1mad, "people who just want to get around without turning into a Cat 6 racer" can do it on seven gears without giving up that much.

But I'm just talking about 7 gears, not a 7-speed freewheel, which are two different subjects. Regarding the freewheel there may be issues with spacing, the size of the cluster, which relates to dropout width and it's lateral position in the wheel. If you start with just the 8-speed hub (even with 7 or fewer gears) those issues won't exist. I suspect that's why people say to avoid the 7-speed bikes - they might not be very compatible with most of the standard frames, wheels, clusters etc out there.