In article ,
(Andrew Sarangan) wrote:

The paragraph you refer to is FAR 91.173. Interestingly, it does not
say "unless otherwise authorized by ATC". It simply says that you must
file a flight plan. There are no exceptions. However, it does not say
that you must file that flight plan with FSS. One could call ATC, tell
them their aircraft type, equipment and destination, and you are
considered to have filed a flight plan.

FAR 91.169 lists all the information you must provide in a flight
plan. This is where it says "unless otherwise authorized by ATC". So
they could waive the requirement to file an alternate, or fuel on
board, people on board etc.. This is what the ATC is waiving every
time you get a 'pop-up'.

This was my impression. Correct me if I am wrong.

Ok, you got me.

The point remains the same, however. It's less disruptive to the
traffic flow to handle you as a pop-up under radar coverage than as a
non-radar IFR departure. ATC can't refuse to handle you as a non-radar
departure, but he can try to bribe you into agreeing to do things his
way :-)