Yes, I use Joel's "rules" to consider mergers all the time. In addition to the subjects Jeff mentioned (off the top of my head):
- There are numerous "music" proposals that probably need to be one or two (maybe three) sites;
- Tons of academic disciplines that can be combined into university-style departments;
- A bunch of fitness/exercise/nutrition proposals that need to be rolled into one;
- More technical proposals than I care to count;
- and many, many, many more.
If you look at the first few pages of up-and-coming beta sites, there are almost always other proposals that would be well served by combing forces with them. That has become an on-going process and I am slowly working my way through them.
Area 51 is brilliant at vetting great ideas for sites, but those ideas don't always work well with each other. Until we can add more abilities for proposals to work out those issues amongst themselves, I take your suggestions and flagging under advisement and I make the more-obvious judgment calls — close and integrate blatant duplicates into existing sites; merge proposals that are spread so thin that none of them would every be created separately; and where that fails, I can contact proposal supporters and work out the most reasonable course of action.
I try not to rush to judgment. I don't generally delete duplicates between early proposals; each has a right to compete. I try not to make judgment calls about a subject's viability; Area 51 vetting works brilliantly at surfacing the best proposals. If there isn't a problem with the initial proposal, I don't generally see it again until the subject reaches the Commitment phase. It's during Commitment that I can take a closer look and see if mergers and joining forces with related proposals can make the resulting site that much stronger.