I am perfectly happy with a custom DTD that I developed and use with structured FrameMaker. However, DITA advocates say go to DITA for reuse. Is that really proven? for technical writing?

One of the benefits with using DITA vs custom DTD, is that DITA is a standard. Which means that people or a CMS systems knows what to expect.

Implementing DITA does require effort, and if reuse is your primary goal I would weigh that against the system you already have.

Do you have a good use case for reuse? I've only ever used it a few times and it has burned me almost every time. With just a few talented writers, it can be managed, but as you add writers, turnover, and complexity, it's very easy for someone to inadvertently change something without understanding the impact. For example, we had a section that was used to introduce multiple similar books. Unknowingly, someone went in and replaced all the general references to something more specific. This eventually got propagated to all the other books. You could easily do the same thing in traditional Framemaker, and there's nothing about DITA that makes this less dangerous.

The real strength of DITA (or any standard for that matter), is that it has out of the box support with many tools and editors. If you decide to switch editors or viewers at some point, you'll have to teach them your proprietary DTD. Depending on the complexity, this can be painful. If you were using DITA, you would only need to teach it any specializations you've done (if any).

DITA is just one flavour of structure – whatever is working for you, go for it. There’s lots of discussion about DITA on the web – surf away ;>)