I disagree. And it's one of many many things why I like eLinks over Links1/2/Hacked, w3m or other text-mode browsers (defined in Options: Protocols->URI Rewriting->Smart Prefixes). Sometimes one facility for multiple different functions is indeed a Good Thing.
There is little reason to limit browsers main text based access field to one access method (URI's) - especially if there's a way to limit what you can do from that field somewhere in browser setting (not sure, but I recall seeing such settings in FF). I've optimized my browser for quick keyboard only access (I also have add-on for mouse gestures, the point being avoiding having to change between the controlling devices unnecessarily) with such add-on's as Keysnail and Mouseless Browsing, yet I don't even remember the hotkey for search field (I only use it to select "Manage Searches..." or to add new search engines) as searching is way easier done from the general access field, prefixing the phrase or URL (latter comes handy when quickly accessing translate.google.com for current URL with C-l C-a t [space] [return]) with letter or short word for engine/site to search (s->startpage.com,g->google.com,w->wikipedia(en),wfi->wikipedia(fi),sana->sanakirja.org(multi-language dictionary),y->youtube, etc.).
I also like how it can list bookmark entries matching the word(s) you type, often giving access to what you want faster than through bookmarks menu - which I have naturally sorted into subfolders up to reasonable level, but that doesn't help at all when what you're looking for could reasonably exist in more than one subfolder; which is where matching your words to bookmark/subfolder names and keywords comes very convenient and quick.
I use that field for those three reasons, I know when I'm using it to type URL or search or to find bookmarks, and the browser knows it well enough to present what I'd expect in dropdown list just perfectly. God knows what could there possibly be that I would find better in usability (THE keyword) with having to use three separate fields for each access method but I'm sure that, with proper settings and possibly add-on's, it could be done with FF if someone likes it better that way. As it is now, I don't know what there is to complain in the way it currently works, even if you don't want to use the field for searching: Enter the URI you want and off it goes anyway. Only thing that annoys me is not knowing another way to access "Manage searches..." than through search field's drop down menu, which unfortunately keeps me from removing the separate search field from the UI alltogether.