First, a quick point of information on terminology. We are using ''pop-up" to denote a cold call to ATC asking for an IFR clearance which had not been filed. That's definitely its common usage by a long shot. But as used used in the "dot 65," the controller manual - it appears once - it refers to any VFR aircraft requesting an IFR clearance, even if previously filed. So really, it refers to both situations. That can be confusing in dcussions.
When you ask for a cold call pop-up you are basically asking ATC to to do your filing for you. Simple enough if the flight is local and no other facilities are involved; more problematic when the flight is longer. Imagine cold-calling in the air in Kansas, asking for am IFR clearance to Montreal. It's really not their job to do your job.
"Need" suggests to me some kind of a problem where a denial results in a problem - "Unable to maintain VFR" is a good one bound to get their attention..
Personally, I've asked for cold call pop-ups maybe a half dozen times and never been denied. But I heard a denial. I was IFR climbing out of Denver. I heard another aircraft calling to open a flight plan they had filed. TRACON replied they couldn't find one on file, so the pilot asked for a pop-up. ATC's response was "unable. Suggest you contact Flight Service."