I use profile for Thunderbird in several different manners. First, there is my Mozilla stuff; I would like to collect all my Mozilla-related messages, etc. into one place. Second, there is my Usenet persona, which is different. Then there is school, and finally other personal relations. So I should (ideally) have four identities each representing how I want to appear. When I write a message, I should be able to select which identity I want to express.
Now how does it work right now? I have 9 identities--each new account I create (4 email, 5 news) creates its own identity. Deleting them more or less requires me to go into my prefs.js (or about:config) and pruning them. Oops. The compose window right now is probably confusing (I have four email addresses selectable but 9 entries in a drop down saying "From"... hmm...), but then again, compose needs its own basket of love. Finally, simultaneously posting to a newsgroup and emailing is... hairy, to say the least. The normal case is at best confusing or redundant while the abnormal case is infuriating.
There's a third piece that needs consideration with identities&emdash;account type extensibility. RSS accounts in the future may want to allow you to comment on posts&emdash;or possibly even write new posts yourself. Similarly, other account types will probably want to have some way to let users write in a generic, HTML manner, what composer is good at. The difference here is in means of transmission.
From a user standpoint, all of this can be done in easy steps. Account creation wouldn't create new identities for each account type, but would allow you to specify an old one or have you create your own. For extensible account types, all that has to be done is to mix in your own compose stuff, present the necessary UI (if any), and the necessary backend hooks. Posting to, e.g., both news and email is as simple as adding a registered newsgroup header (i.e., Newsgroup) and adding a registered email header (i.e., To, CC, or BCC) in the compose window.