First off... if there do charge a fee, it will most likely be much more than $20 a year. More like $10/month.
But even then, i wouldn't. With the way Napster works, you're voluntarily sharing your files to access other people's files. You're giving out your bandwidth so others can download your songs... so why should I have to pay for this? If anything, Napster should be compensating me for putting my bandwidth and storage space out there for people to use.
Now, if they had their own servers serving songs on their own bandwidth, then i would.