While I agree that concern with passers-by memorizing or writing down a session ID in the URL is excessive (especially since most sites' session IDs are tacked onto the end of a URL that's large enough that it isn't all displayed on the screen anyhow), putting it in the URL also means that it will be recorded in the server's access log as well as any intermediate proxy logs. Those logs may be readable by untrusted third parties, creating a possible threat. (I wouldn't go so far as to call it a likely threat, but it's nowhere near as remote a possibility as someone looking over your shoulder and getting it off the screen.)
In any case, like you, I've pretty well taken the position that cookies are a fact of life if you want to log in to a site, so just enable them (but disable third-party cookies if your browser has that option) and get on with it.
