IGC rules have fixed proportions of tasks
No, they don't. They recommend but do not require no more than 2/3rds of one task type.
6.1) TASK TYPES The following task types are available for use during the
Championships. A single task type should not be used for more than 67% of the
Championship Days in each class.
Preliminary Remarks b) In this Annex the words "must", "shall", and "may not" indicate mandatory
requirements; "should" indicates a recommendation; "may" indicates what is
permitted; and "will" indicates what is going to happen.

https://www.youtube.com/watch?v=6GMkuPiIZ2k

I am not familiar with the US rules, but I do know the IGC rules quite well and they're not the bogeymen they're made out to be. Nor are they perfect - still quite a few step functions left. I would like to see them gone, like Bruce Hoult suggested.

On Wednesday, December 16, 2015 at 2:50:34 PM UTC+11, John Cochrane wrote:
Sean:

Again. (And again, and again, and again): No country uses IGC rules. IGC rules, verbatim, are only applicable for world championships. Start right at entry is by nomination from National Aero Clubs. So how are you going to do entries for US nationals if you "use IGC rules?" Ask the NAC to decide who gets in to Nephi? And go on from there. Stewards, international jury, site selection, so forth. You simply cannot use these, verbatim, for national and regional contests. Every country creates a set of national rules, adapting IGC rules more or less.

Again. And again and again and again: The choice of assigned tasks vs. turn area tasks vs. MATs is completely up to the competition director. IGC rules have fixed proportions of tasks -- resulting in assigned tasks in thunderstorms because we used up the allowed fraction of turn area tasks. US rules do not have any fixed proportions. THIS IS ENTIRELY UP TO THE CD, not the rules. If you want assigned tasks, talk to your CD. Talk to your fellow pilots at nationals and convince them that's what they really want to do.

The clamor for "use IGC rules" would be much reduced if anyone bothered to actually read the rules before clamoring.

John Cochrane