skyocus, I have to disagree. When you are VFR-on-top on an IFR flight plan, you are no longer afforded IFR separation. If IFR separation had to be applied, there would not be a single operational advantage I could think of for a pilot requesting a VFR climb.
Since haven't used a "vfr until" clearance I reserve the right to be wrong, but I was thinking the same thing: there doesn't seem to be any advantage for the controller. If you look at the paragraph it states it only to be used at pliot request or for noise abatement so the conditions are very limited. For the first condition it sounds useful for a pilot that wants to stay in good wx but stay IFR. It doesn't seem to help the controller out. I've never had to work under strict noise abatement rules so I'm not sure how the second condition applies.
VFR on top is something I haven't had to use before either, but going on memory the phraseology is "Climb to and report reaching VFR on top. If not on top by XXX maintain YYY and advise." That means to me once the pilot reports "VFR on top" they are maintaining VFR (on top). Compare that to: "MAINTAIN VFR CONDITIONS
UNTIL (time or fix)". or "MAINTAIN VFR CONDITIONS
Keep in mind though airport operations and LOAs may prevent these procedures from being applied.
That tracks with what I'd heard about Caldwell. What I truly don't understand, though, is the spirit behind such an LOA.
Case in point, you can be stuck at CDW waiting for 15 mins for an IFR release while they launch jets out of nearby MMU, and props out of N07 (Lincoln Park, non-towered). If it's a nice day, why NOT let the IFR guy launch out of CDW with a VFR climb if he says he can do it.
Instead, the pilot is literally trapped on the ground, not allowed to go. If he does opt to launch VFR, the LOA (apparently) states that the IFR plan should be REMOVED from the system. I just don't get it.
It's actually EASIER to depart from N07 under such circumstances (I do VFR release on IFR flight plan 1/3 times).
I would call the tower to try and find out why they remove the flight plan when an airplane departs VFR. The only reason I can think of is to keep the radar controller from getting swamped with VFR a/c airborne requesting IFR clearance. If you're busy and 1 or 2 planes depart VFR requesting IFR that's one thing, but if all of a sudden you have 5+ that can be a headache by its self.
My original question, though, is whether ATC was happier to have a specific constraint on the VFR climb (ie. "I can maintain VFR until reaching 4000", or "I can maintain VFR until [first fix on flight plan]") versus a generic "I'll depart VFR"). It seems like it's not anymore helpful, though, as the reason for the denial is not for lack of specifics, but more to do with the presence of an LOA which prevents the action flat out.
Assuming "maintain vfr conditions until 4000" can be used to side step IFR separation until you reach 4000 I would think some controller might balk at it because it kind of handcuffs them. When you simply depart VFR the controller can wait until workload permits and they have already established the necessary IFR separation. With "maintain vfr until" it takes both those factors out of the controllers hands.