I haven't played with last fm so I can't say. But, despite all the issues with myspace, I think it's a nice tool for artists to use if they desire to. Especially they already have a private website. The amount of people that spend their entire day on myspace just messaging and commenting people while looking for new media is pretty crazy. So, even the completely unknown end up getting some fair exposure through it.
It can be set up with very little effort. There are "template generators" all over the place which pretty much remove the need for any constant HTML updating. For a musician, you can basically get away with just having the main information point to whatever website you are using as your most frequently updated page. From there, on the myspace side, you can pretty much just bounce in every so often to add some new tracks, and check friend requests, messages and comments.
Basically, if you're simply using it as a means of promotion, and not personal communication, it's fairly simple and effective. And, in the end, it costs you nothing but a little time. I feel the same applies to just about all "free" promo services.