I hope I am not nitpicking here, but I was concerned by your comment that the gate agent "...just puts you there...". You may know this, but be aware that just because there are empty seats available in C or F, gate agents will not start moving passengers up using operational upgrades. In situations involving operational upgrades, economy class is oversold and some seats are empty in F or C. Generally, if a gate agent needs to "free-up" some seats in economy class, she/he will upgrade passengers according to each passenger's frequent flyer status.