Tell you’re climbing partners they’re full of it, Mike. And read my lips: “It…doesn’t…rain…on Mount Rainier….in July.” At least not often. Despite Washington State’s wet reputation, the simple fact is that we are in a drought from about June 15 through October 15. Yes, the odd storm will sweep through - one hit mid-July last year and lasted a week - but that’s the exception, not the rule. In any event, any big storm also will be a big COLD storm, so you will have snow at 10,000 feet and up, not rain.
So here’s the deal: One, either a down or synthetic bag will be fine. Get one rated to between ten and 20 degrees. And you should be OK with open-air bivys. I guess I’d recommend a light bivy bag, whether using a down or synthetic bag, just because the extra shelter could come in handy.
Two, check the weather forecast, and don’t be shy about packing a tent if it’s marginal. Rainier is a big, Alaskan-style mountain, one that creates its own weather. And while my bet is that conditions will be fine - clear and sunny, with high winds above 12,000 feet the worst problembig storms can hit.
As for synthetic-bag durability, it’s true that short-staple fibers such as Primaloft supposedly do not last as long as long-staple fibers such as Polarguard. But in reality, good manufacturing goes a long way toward overcoming this problem. Integral Designs bags are fine. A good one for your trip would be the North Twin ($220), a really nice, ten-degree bag.