I recently listened to a podcast of Scott Hanselman talking to Joel Spolsky.
One of the things they talked about was how Fog Creek (Spolsky's company) wrote their own compiler because it was easier than rewriting their application, which was written in an ancient version of VBScript.
I've always been a little defensive about how my company writes their applications in our own language (Suneido). At best people look at me funny, at worst they think I'm crazy. Our salesmen have to skirt around the issue because it scares people to hear you're not using big name tools. As the old saying goes, "no one gets fired for buying IBM". Nowadays you can substitute Microsoft or Oracle or Java.
So it was heartening to hear Spolsky talk about (aka defend) why they wrote their own compiler. One of his points was that writing a compiler is not that hard. People tend to think it's a big deal, but for a smallish language it's not that big a job. For example, Suneido's compiler makes up a tiny fraction (less than 1%) of the total source code in my company's main application.
I'm the first to admit that part of the reason is simply that I like developing languages (and database servers, and IDE's, and frameworks) better than I like writing applications.
But there are other reasons. Having your own platform has its costs, but it also has its benefits. It's a good way to insulate yourself from the fast pace of change in the computer industry. My company still has customers running software that we originally wrote on MS-DOS. The exact same application code is now running on Vista. There are not many commercial platforms that can claim that. Even things like Java that are supposed to be stable platforms change a lot over the years. And Microsoft changes languages and frameworks faster than you want to keep up with. (Spolsky's problem.)
Of course, that can mean you're not using the latest bleeding edge technology, but that's not such a bad thing for a business application.