When making a character for an RPG it can be hard to accept that you want to have weaknesses in your character. The impulse is to make someone who is self-sufficient and able to take care of themselves. The desire is to have a character like the main character in a book. It makes sense when you think about it. Most characters are adventurers who make their living travelling on the road. It only makes sense that they'd be able to take care of themselves, right? Well, maybe not. At least, not if you want to get the most out of your RPG experience.
The Danger Of A Jack Of All Trades
The danger in playing a jack of all trades comes in the second part of the phrase, "master of none." See, when your character is able to do everything - but has a limited pool of XP with which to acquire those abilities - you end up being mediocre at many things instead of great at a few things.
Think of it like a D&D party. D&D, by being a class based RPG, gives each character a specific set of strengths and along with those strengths defined weaknesses. As such, a properly made level 10 party of adventurers has a fighting/tanking specialist, a spell slinging specialist, a rogue specialist, and so on like that. However, when your character is a jack of all trades instead of being that level 10 specialist for the group, you're closer to being a level 6 specialist who can also adequately cover the jobs of some other party members at a 4th to 5th level capability as well.
This isn't a problem in and of itself, but when multiple people in a group do it then you have issues. Why? because if everyone in the group is 10th level, but can only act as if they are 6th, you're going to get wiped out in the first "fair" fight that comes up.
A Defined Job
Building in a defined strength gives your character a defined job. This gives you an area that you 'own' in the group. In other words, it gives you a place to shine.
You want to hold onto this place, because it is the area your GM can put you for you to have your key moments. Want that crowning moment of bad assery? Odds are it is going to come from the area you've made yourself strong.
However, being strong is easy. Being weak is hard.
The Benefit To Having A Weakness
There are two key benefits to having a weakness on your character. The first is for the group, and the second is for you yourself.
The benefit to the group is simple: by having a weakness you leave room in the group for someone else to be strong. If everyone in the group does this, then everyone has a place to be strong and a place where they allow the others in the group to be strong for them. It benefits everyone and makes for a nice strong group that covers for each other.
But I spoke of a benefit to you too, didn't I? The benefit here is that by having a place of weakness you have a place to allow drama and tension to come in. There is little drama to be gained from always being challenged where you are strong. However, a good GM will see this area of weakness and use it to bring turmoil and tension into the game. You want this, because turmoil and tension means plot for your character, screentime for your character, and fun for you.
Good RP can't happen without bad rolls, and a character can not rise above conflict if she is never put in a bad spot. By placing these weaknesses you not tell the GM where you want to be challenged and where you want trouble coming from, and where you want to be strong and feel like a super star. This can't happen without a defined strength, and a defined weakness.