Yes, I do think they helped. Before HTML 2, validation didn't exist, so whether or not a document's markup made any sense was merely an opinion. "Best viewed with" was the order of the day because there was no way to know whether a browser would handle a document reasonably other than testing them together.

The web mostly interoperates now, but if we eliminate validation with HTML 5 I fully expect a return to the trainwreck we faced in 1995.

First, not having DTD is not the same as not being able to validate: there is a validator for HTML5, validator.nu. You can also use XML serialization of HTML5 and XML tools to make sure your document is valid XML.

Second, having means to validate markup does not mean that authors will care and do that: what portion of document having doctype with dtd slapped on top are actually valid? Good markup is not produced by tools, but by those who care.

Thanks! http://about.validator.nu/#pitch seems to be using a RELAX NG schema instead of a DTD, which is fine, though it's not clear where the schema actually came from. The draft alludes to "Criteria that cannot be expressed by a DTD, but can still be checked by a machine", and that's a good sign. But I can't find any formalized list of those criteria, and that's really not.

And sure, there will always be overworked or ignorant authors who roll out slipshod work. But I at least want it to be possible to expect better, as it was not before we had the first DTD for HTML.