Which I think is a major factor here. Most people I've seen debate conversions of other fantasy systems have resorted to VPPs in order to convert the magic systems.
Personally, I'd define magic VPPs much like Gadget Pools in Champions- not a "pool of do anything you want", but a pool that covers a lot of miscellaneous capabilities that it is no longer cost efficient to represent with separate spells or Multipowers. That is, if you've spent so many points on Multipower slots that you might as well convert them to the Base Cost of a VPP, then that's the time to do it.
IMO, a beginning character shouldn't have a pool, and a beginning HERO player shouldn't have one until he's familiar enough with the system to know how it works. In story terms, most beginning mages know a few separate effects. By the time the character is skilled enough to channel effects at will (Rand in Wheel of Time, Willow in Buffy the Vampire Slayer, etc.) the character in game terms will have enough experience (and Experience Points) to make the transition to VPP worthwhile AND justified in terms of story background.
If VPPs seem too powerful, remember that they normally require an extra action and a Skill Roll to change over. For most fantasy (i.e. simulating D&D-type stuff) the 'Cosmic' Advantages shouldn't be allowed. Most casters can't simply make up spells on the fly; 'arcane' casters need to consult their books to change the VPP, meaning also that they are limited to the spells that they've actually researched- i.e. only those the GM allowed him to have or that he made up with Inventor/Spell Research (ultimately, the same). Clerics would only have those powers their god allowed, meaning that they're even more under GM control.
Basically, in HERO System, with great power comes great responsibility. The GM has more freedom to design the parameters of his game system than with any other rules, but that also means weighing how things would work without relying on some designers' notion of "game balance."
Hope this helps.
JG