If lesser extensions are still available it is harder to get a high sale on the .com because at some point the buyer is going to take the .net for $8 instead of paying you four or five figures for the .com. It all depends on if you're talking about the true value (earning potential, memorability, etc), or if by value you mean what someone is willing to pay. Having the .net available doesn't make the .com any less memorable or useful.
If you're talking about resale value, if the potential buyer has reasonable alternatives (.net, .org, etc) then you lose a lot of bargaining power. You want to eliminate that. Look at the sale of iReport.com, when Schwartz heard CNN was doing the whole iReport thing, he registered other extensions so they didn't have an easy out to paying him a boatload of money... he even got i-Report.com just to be sure. Does that mean it's always smart to register every gTLD of a domain? Definitely not, it really needs to be assessed on a case-by-case basis.
I would agree with Brad though, if the .com is really that great, it will likely already be taken in other extensions.