Automatic spam filters and search engines are going to down rank them heavily. It's hard to automatically evaluate domain names such as xkcd but anything under .ecom is probably trash.
Anybody could have any number of reasons for wanting a particular domain. Maybe someone will make the next del.icio.us? Maybe a TLD you don't like completes a word or phrase that makes a good brand, or is part of the next big viral trend that goes on to shape our culture in ways we wouldn't have anticipated.
Maybe people are just okay with being penalized by search engines because that's counterbalanced by other objectives. etc.
A more useful option IMO is to have things default to .com if someone does not enter a domain name like how you can avoid country codes or area codes when dialing long distance.
However, by adding a huge list of TLD's your removing that option. Even worse while .edu means something WTF does .cow or .zzl mean? At best there is simply going to be another .com land grab except now seeing yourbank.bank or yourbank.bnk does not mean it's actually yourbank.
So yea, I do think it's a bad idea.
They don't need a reason to be added, other than offering more choice and more possibilities. You can not like them all you want, that doesn't mean they won't have value or that people shouldn't be free to explore the possibilities as they please.
Let's hope not. I could never remember where that first dot went and usually took at least a couple of goes to find the site. Thankfully they can now be found at the less 'look how clever we've been', but infinitely easier to remember delicious.com