Tough question, and well asked.

My gut where I'm at now (subject to change) is that I have to be good enough at everything not to get *obliterated* by anything. But beyond that, focus where I'm best.

In our rolling last night we played a game on the ground where one guy was on defense, the other guy on offense. The burden was on the offensive guy to tap his opponent in the round. If there was no tap, the defensive guy stayed on the mat. We found that the advantage was so much in favor of the defensive guy. It was way easier to ball up and defend than it was to tap a guy who was only worried about defense. So can we say defense is "easier" than offense? If so, I just need to be good enough to defend myself in my weak areas. But in my strengths, I want to be able to pick apart even a defensive opponent.

The tough question is then, what's good enough? Maybe the 80-20 rule is a good one. Eighty percent of your effort and training on strengths, 20 percent on weaknesses. Just a starting point.

Preparing for a specific opponent in a sportfight versus training to defend yourself against an unknown on the street might be different. But since I train for fun and self defense, I really want to be "good enough" to defend myself in any range. If the fight ends in a self-defense stalemate, great, in most cases. I can be happy with a draw. But if and when the fight goes to the area of my strength, I'm more likely to win it.

And speaking of the fun factor, if training what's fun to you–usually your strength–is what keeps you training, I say go for it. Whatever keeps you coming back to the mat for more. I've seen way too many people come and go. Longevity, baby. Everything will improve if you just keep coming back. Nothing will if you don't.

In my opinion focusing purely on making your weakness better often doesn't help rather it hurts. Because you are more willing to play with areas that you are weak in and your competitor will take advantage of.  However indirectly building on weaknesses is beneficial. For example if a fighters strength is striking, but his submission game is weak and you directly try to teach the guard game for submissions. Then his game will ultimately suffer, because his striking skills tell him NOT to be on his back and to get up or get on top. Yet his BJJ game which he just learned is not developed enough but is emphasized in training creating a metal distraction and ultimately weakening his game. However working indirectly will improve his game! Training his striking hard, as well as his take down defense, while teaching him subs off of what he is good at, takedown defense.  For example a rolling omoplata off the single leg defense. 

Me personally I am a striker, but then I started training BJJ and a little bit of CACC and CSW and I started getting taken down much easier.  Because I felt more comfortable on the ground I wasn't fighting as hard or defending as well as I had when I had only done striking.