At the end of the day, search is a locator service and needs to be complete. If the analogy is that search engine's are the new Yellow Pages (which they are), a vertical search engine is the equivalent of a different Yellow Pages book for each type of listing. Do you want a separate Yellow Pages for auto dealers, restaurant, health care, etc.? Clearly not. As a locator service, search engines don't generate casual browsing. People don't go there for fun - they go their to do something.
Content sites on the other hand are the exact opposite. They're a destination - a place you actually go to spend time. Casual browsing is their entire audience. It seems that that for a whole lot of ways - branding, monetization, audience, etc. - content sites work best as verticals. If search engines are the Yellow Pages, then content sites are the magazine rack - and the magazine rack is always verticalized.
So with that, are their any crossovers? Are there any search sites that are doing well as verticals? I can think of two: Itunes (for music) and Amazon (for books). You might argue and say those aren't search sites, but rather e-commerce sites. I think they're vertical search engines with e-commerce monetization. Is that an important distinction? Actually, yes.
As noted above, one of the problems search engine's have is they don't gather a casual audience. This is especially true of verticals. Rich has a rant about how Google has to do a good job on non-monetize-able queries (like "Mike Markson" - no one buys my keyword) precisely so they can gather an audience for the monetizeable ones. This is why sites like shopping.com have to re-acquire their audience every month from Google. As a search engine, when you ignore a class of query you do so at your own peril - people will know longer think of you as a locator service.
Amazon and Itunes don't have that problem (although I'm sure they buy their share of traffic). I think this is because from a brand perspective folks know that their sites are more than just locator services. They can buy stuff there. As a result people actually do go there for the occasional casual browsing. Call it the cyber-equivalent to window shopping. So it turns out that to succeed as a vertical search engine, you need to do something other than be a locator service. One way is by being the corner store.