Most of the time I get VFR On Top (OTP) requests, the pilot cancels after they reach VMC anyway. If the pilot does not cancel, then although they're able to maneuver VFR, they are still expected to fly their filed course to destination. It's like an IFR flight with deviations constantly approved.
As for requesting OTP as opposed to just filing an IFR flight plan and canceling.... To us ATC it's a pretty similar thing with a few procedural differences.
I could be wrong, but maybe the advantage to filing OTP is that it shows up as the "requested altitude" on our flight strips (it will say "OTP instead of 080 or 100 etc). So instead of getting "expect 1-0 thousand 1-0 minutes after departure," you get "expect climb to VFR On Top at or below <that sector's altitude limit> 1-0 minutes after departure." So if you don't know where the tops are, you'll be authorized (traffic permitting) to climb up to that sector's highest altitude in an attempt to get above the cloud, and you need only report when you finally break out.
The alternative is filing an IFR flight plan with a final altitude you THINK is above the clouds. But then, if it isn't, you'll have to request higher and higher with the controller mid-flight until you get there. In practice I see it done both ways and it usually works fine.
4-4-8 of the AIM also explains a pilot's responsibility when operating "on top"
Some pertinent sections:
e. When operating in VFR conditions with an ATC
authorization to “maintain VFR-on-top/maintain
VFR conditions” pilots on IFR flight plans must:
1. Fly at the appropriate VFR altitude as
prescribed in 14 CFR Section 91.159.
3. Comply with instrument flight rules that are
applicable to this flight; i.e., minimum IFR altitudes,
position reporting, radio communications, course to
be flown, adherence to ATC clearance, etc.
NOTE−
Pilots should advise ATC prior to any altitude change to
insure the exchange of accurate traffic information.
