If I own the .com, and I'm not interested in developing a site, is it worth buying the .net if it available? Will it increase my resell to offer .com,.net,.info?
You could do that, but it would be pretty pointless to get the .net aswell if you aren't developing it, but I would get the .net if it was a good domain too.
what jakeruston said is true. No point if you are not developing. But for few $ you can grab and keep it. Who knows domain buyer like to buy both if he serious about domain name / developing site. If I were you, and if the good domain word I would keep .net if I get at regular price.