Hi, ego, I'd say there are a couple of issues involved. Not only must you consider the file extension, but how does one get to the page? I've seen link directories that require the user to use a search box - these links almost certainly will never get spidered. Others may have links from an index page that include a query string - if they are a hard link on a static page (or an apparently static page) these have a greater probability of getting spidered.
You can always do a Google advanced search to determine whether a particular site's links are getting spidered.
Also, don't forget about the value of good link in generating direct traffic. I'm all for high page ranks, but I've obtained links from sites with average or below-average page rank that generate hits every day. I'd be far happier to have 500 visits a day coming from links on a hundred or two hundred other sites than from a #1 position that could get blown away overnight due to an algo change, a database error, a poorly tuned spam filter, etc.