I think there are many good reasons to not use an ORM. First and foremost, I'm a .NET developer and I like to stick within what the wonderful .NET framework has already provided to me. It does everything I possibly need it to. By doing this, you stay with a more standard approach, and thus there is a much better chance of any other developer working on the same project down the road being able to pick up what's there and run with it. The data access capabilities already provided by Microsoft are quite ample, there's no reason to discard them.
I've been a professional developer for 10 years, lead multiple very successful million+ dollar projects, and I have never once written an application that needed to be able to switch to any database. Why would you ever want a client to do this? Plan carefully, pick the right database for what you need, and stick with it. Personally SQL Server has been able to do anything I've ever needed to do. It's easy and it works great. There's even a free version that supports up to 10GB data. Oh, and it works awesome with .NET.
I have recently had to start working on several projects that use an ORM as the datalayer. I think it's bad, and something extra I had to learn how to use for no reason whatsoever. In the insanely rare circumstance the customer did need to change databases, I could have easily reworked the entire datalayer in less time than I've spent fooling with the ORM providers.
Honestly I think there is one real use for an ORM: If you're building an application like SAP that really does need the ability to run on multiple databases. Otherwise as a solution provider, I tell my clients this application is designed to run on this database and that is how it is. Once again, after 10 years and a countless number of applications, this has never been a problem.
Otherwise I think ORMs are for developers that don't understand less is more, and think the more cool 3rd party tools they use in their app, the better their app will be. I'll leave things like this to the die hard uber geeks while I crank out much more great software in the meantime that any developer can pick up and immediately be productive with.
