Normally I'd say no, but it really depends. Here are some situations and what I'd personally do:
Scenario 1 -
You're a new band and have a couple demo songs. In this case, you've gotta decide on a few things. On the one hand, you've got nothing to sell, and nobody knows about you, so it probably doesn't matter. On the other hand, if you're a new band, you're starting out even with the 5 million+ bands out there that are trying to stand out/be heard. Anything you can do to get people to listen to/remember your band is a good thing at this point.
Scenario 2 -
Your band has their first CD/EP out and want to promote it What my band did was decide on a single, and make that one downloadable, along with making one other track (out of 4 in this case) listenable, but not downloadable. Having a single that everybody can always download and listen to does several nice things for you. It makes that song seem special, so people will eventually start asking for it at live shows. It also lets people put the song on his/her iPod/computer and listen to it whenever they want/show their friends/etc. If the song's good, people will eventually buy your CD because of it as well.
Scenario 3 -
Your band has been around for a long time and is putting out a 2nd or 3rd full-length CD I'd make some old songs downloadable, and have your new stuff be just listenable in this case. If you have an established fanbase, you won't have as hard of a time selling your new CD so there's not as much of a reason to have the one song be downloadable. If you know there's basically one good single and 10 mediocre songs, it's imperative that you don't make the one song downloadable, as you've removed the reason for people to buy your CD.
These are just my experiences in my band (
www.myspace.com/everestband)
