It sounds to me like you already built your own IoC container (using the various patterns which were described by Martin Fowler) and are asking why someone else's implementation is better than yours.
So, you have a bunch of code that already works. And are wondering why you would want to replace it with someone else's implementation.
Pros for considering a third-party IoC container
- You get bugs fixed for free
- The library design may be better than yours
- People may be already familiar with the particular library
- The library may be faster than yours
- It may have some features you wish you implemented but never had time to (do you have a service locater?)
Cons
- You get bugs introduced, for free :)
- The library design may be worse than yours
- You have to learn a new API
- Too many features you will never use
- It's usually harder to debug code you did not write
- Migrating from a previous IoC container may be tedious
So, weigh your pros against your cons and make a decision.
