Instead of copying them into a new playlist of your own?
1, I update my playlists constantly. New releases or newly available recordings in a series, new works that fit into a themed playlist, I add them once I find them on Spotify. For example, for the Michael Gielen Mahler cycle playlist, the Symphony No.2, 3, 8 were made available on Spotify after I posted that playlist, and Das Lied von der Erde was released just recently, I updated that playlist accordingly. If you subscribed to it, all those newly added recordings will appear automatically.
2, And if you use Spotify Social, your friends may discover my other playlists through your Spotify profile once you subscribed. You can help me spreading the music by simply doing less. Clicking Subscribe is actually easier than "select all, copy, create new playlist, input name, paste", isn't it?
3, You can also browse other subscribers' profiles, to look for their interesting playlists. I discovered Harry Partch through one of the subscribers mreddington.
1, You cannot update the playlists yourself (except for collaborative playlists) . 2, Cannot change their names either.
Unless you do that a lot, I suggest you to subscribe. You can always leave a comment to suggest a update for certain playlist that I overlooked, or just send the recording you want to add into my Spotify inbox. Since all playlists are stored in the Spotify servers, I don't think there will be any performance difference (loading speed, etc.) between subscribing and copying into your own playlists.
And here's how to for those of you who don't know (believe me, one of this blog's most ardent supporters just told me that he didn't know he could subscribe). OK, click a playlist link: Radiohead Office Charts, to open it in Spotify, as you see in the pic above, to the right of the playlist image (a mash-up of album sleeves most of the time), under the playlist name, in this case, "Radiohead Office Charts", there is a "Subscribe" button, click it and, done. By the way, the Radiohead playlist is a great mix of classical, world music and contemporary pop, and updated regularly, well worth subscribing.