On at least some commercial controls (Bosch CNC220 and Siemens Sinumerik for example) It is possible to set the G0 feedrates from the program. There was some discussion as to whether a G0 move should ever be done at anything other than the maximum feedrate, which, by definition, is what it should do normally. However, at least some controls do allow modification of the G0 feeds from the program, and some users do find it useful.
For example, on a CNC punch, typically, the sheet of meterial is moved, the punch head stays still. G1 is used within parts, G0 is used to position the sheet from part to part. You may not want to move a sheet of 10mm steel at the same rate as a sheet of 2mm steel (although, jmk did point out that was an accel problem, not a feedrate problem) or, on a very thin sheet, say 0.4mm you might not want it to G0 too fast, to prevent it flapping about like a sheet of paper.
Many examples on other types of machine no doubt do exist. It is a feature implemented on other controls, so it probably something at least some users need or use.
One thought was to allow F words on the G0 line,
G0 F50 X0 Y0
G1 F10 X10
all the G0 moves on a Bosch control would occure at feedrate 50, the G1 moves at feedrate 10. In effect, there are 2 feedrate variables, one for G1, one for G0. The problem is what happens when an F word is encountered on a line on its own, should that apply to G1 feedrates or G0. On Bosch it applies to whatever the current motion mode is. if we are in G0 it affects G0. at the head of a program the motion state is ambiguous, unless a G0 or G1 has been set, the mode is unknown, so an F word could apply to either.
A suggestion from jmk is that anf F word affecting G0 should be allowed on a G0 line ONLY. This would not break any current programs (F words on the G0 line are illegal), and it would provide a facility to alter G0 feeds on the fly from the program.
Later: cradek says that F words are currently allowed on G0 lines, and this changes the feed rate for upcoming G1/2/3 moves. Changing the behavior of G0 Fxxx to do the above breaks existing legal ngc programs and is therefore not a good solution.