I don't agree with that one.
A PG can perform very well with only HD, OD and PS. Boosting the flow and the defense.
Imo SG uses the most of DR, JS and JR, and sidestats HD, OD and PS. A PG visa versa. Where we can take HD and DR almost as 1 skill imo since 1 always trains the other 80%.
A PF needs the mainskills IS, ID and RB and uses SB, JS and some OD. I got the feeling C's are taking more FT's as well, although I can't really prove that, not enough information, just the feeling. I'm still not completely sure if a C uses JS too, but I guess so. This would be IS, ID and RB and SB, JS, x for C's.
I don't think the BB's designed it 4vs6 (which imo should be very strange if they really did) but 3+3 vs 3+3. It might be that the 3 of the C's are a little less important than the ones of guards but I just don't believe the statement that C's only need 4 skills and Guards need 6.
Guards can do very well with the right 3, just like C's and PF's.
Guards are just trained more overall most of the time (the might come due more secondary training there as well), but I think when a C is trained at 6 skills, including JS and OD they are even as effective at a certain point than a C with only 3 high skills.
Now you see C's being salary beasts when they are trained at 3/4 skills only, but maybe when they will be trained at 6 they will have around the same salary as the guards, and will be even as effective. If a PG is trained at OD, HD and PS only he will get a huge salary as well, and still perform very well.