...I would say I also think Pend should be pretty good at the game, even if he ain't good at C&C he still knows what the weaknesses and strengths are of the different generals, way better than anyone else
I usually have 3 general styles of play...
Play to Win
Play for Fun
Play to Test
I used to play a lot, and was much better than I am now. I spend a lot more time modding than playing, but you're right, I know every unit and faction inside and out and that makes a difference. When I do play an RTS for fun it's usually Dawn of War or Company of Heroes. I modded DoW for a year while I was in Iraq and had tones of fun. I had a 13 man gaming group and we LAN'ed every night for 4 months.
When I play to Win I usually win, especially 1v1, more than 95% of the time. I don't think it's because I'm a better RTS player, but rather because I have intimate knowledge about the factions. I know what general to use and how to use them. Even in RTS games I don't mod I usually have a strong grasp on the balance just because it's "what I do".
When I play for fun I pick a fight with every other player at the same time and end up fighting everyone.. tones of fun. I'll usually have the highest kills but I seldom win that way. The more players there are the more likely I am to play for fun, and the less likely I'll win... but the more fun it is.
Playing to test... it's a grey zone... sometimes my reasons for playing really start to blend together. For example, the last time I played online vs a guy from the forum he was an early game infantry spammer (nothing wrong with that, it was just his style). I wanted to test Micro General vs early infantry spams because I knew he had a weak spot in the balance... So, I played to Win, knowing I was at a huge disadvantage. Even when I out built him 3 to 1 Micro General couldn't stop the hundreds of infantry... It was a productive test and I started working on an infantry re-balance to effect all generals.
Often I'll play a faction I expect to be at a terrible disadvantage and try to win with it... Its fun, it's testing, and I get to try hard to win but expecting to loose.... knowing I'll win isn't any fun, it's better when it's a surprise.