You may think of having all extensions as being a good idea. (google for example) But for us small guys as long as we have the .com we are kings.. We do not need the .info or .mobi.
Also, if you have the .com and someone else owned the .net and they developed the .net into a site you could approach them to buy the .com
(One more potential buyer that you do not have now because you own all the extensions)
I'm rambling..