In Dungeons & Dragons 3.5, there are 11 base classes:
If you combine the two, as you progress you get a character of average toughness whose attacks don't get that much better and whose magic becomes formidable, but far from terrifying. By trying to get some of the other, you lose the best of either.
The bard is traditionally the most hated class. Why? Because the bard is a "jack of all trades, master of none." The bard has moderate battle skills, moderate thieving abilities, and moderate magic abilities. The bard can do almost everything -- he just can't do anything to a high caliber, even when he's completed all his normal level advancements.
But if the bard is so bad, guess what? Pick 10 classes (you can't have all 11 because of the special rules regarding monks and paladins), raise each of them to level 2, and you'll have a level 20 character, which is the highest normal advancement (before entering epic levels). And this will single-handedly be the worst character of all. They will be able to do anything. But they will suck at everything.
Real life isn't that much different. You can basically extend your expertise and experience in two ways:
- Broaden your knowledge
- Deepen your knowledge
However, if your knowledge, expertise and experience are all in the form of a hole that you're digging, with each stab of the shovel into the dirt, you can either make the whole wider or deeper, but not both (except for your very first strikes, which both broaden and deepen the hole simultaneously). The difficulty here is that depth is necessary.
With depth comes impact. The deeper your knowledge, the greater an impact you can provide.
With breadth comes range. If depth determines the severity of the impact, then breadth determines how many contexts in which you can make an impact.
Too little breadth means specialising in something that is seldom applicable. Too much breadth means sacrificing depth, which in turn means being broadly okayish at lots of stuff, but being rather incompetent when it comes to the specifics. The right amount of breadth allows a great deal of applicable depth.