I wrote my own MVC framework for Coldfusion because the current "flavour of the month" Mach-II was horrendously slow. After switching my page generation time dropped from 2-5 seconds down to 9 milliseconds.
Over the last 3 years I've developed this framework into a rival for any commercial or open-source framework I've used (and I've used quite a few) by building in function libraries and components for a range of common tasks (CMS, CC processing, Image manipulation, etc..)
Although there was no doubt some "re-inventing the wheel" the wheel I ended up with was exactly what I need to do my job. I understand how it works with an intimacy that no documentation could ever provide.
Of course, one day some future programmer may be cursing over my code wishing a pox on me for not using their favorite library - but frankly - I just couldn't care less. I wrote it for ME, it does what I need and it does it well. I also learned a lot in the process.
Having said that you are NOT automatically doing your customers/co-workers a disservice by writing your own framework. Public frameworks tend to have no real direction so they tend to bloat massively trying to keep everyone happy. This bloat means more to learn, more that can go wrong. Your framework will be meeting a much smaller set of requirements and with good documentation could be a lot easier to understand and setup than a more established public one.
I say go for it, live on the edge a little. Maybe in 5 years you'll release the next "Mach-II" or whatever and we can all bitch about it.