I think 

Scott Adams

approach is better for almost everybody because most people don't live in such defined worlds such as 

Bruce Lee

. 

If you're in a sport like martial arts or basketball then the rules/goals are so much more defined than what most people deal with.

Most of us are in settings where jobs/goals/norms are in constant flux. Probably less wise to try to go for mastery in a narrow niche in such context.

However I do think that a lot of people hear Scott's take on this and think it's easy to become average at something. Even though he might be medicore at the things he listed off, I think the combination of these mediocre skills are a skill in itself. And in that combining skill I'd say he has some level of mastery.