I keep my bag the same in all conditions, if you change it when it's rainy, or windy, or any other weather phenomena then you'll lose consistency. If you don't know how to work each of your discs in different conditions then why throw them?
I agree in principle, but in practice, I think many of us have a regular rotation that is probably 30-50 discs. I usually only carry 6-15 discs on the course, but after playing for almost 13 years in all kinds of conditions, I have a pretty good idea what a lot of discs do -- even those that I don't throw every round.
I have the half a dozen or so discs that I use all the time, and will still be most of my throws on a given course in any conditions. But depending on the course and conditions, it is often worth cycling on some other plastic. On an unusual course or in unusual conditions, the discs that cycle in come from those 30-50 discs that remain in regular rotation.
Windy rotation:
PUTTS: For a super windy day, I add CE Rhyno to my bag for short upshots and maybe even putts. I love my Wizards, but a CE Rhyno is a more predictable disc, less effected by the wind. It can save a few strokes.
MIDS: I always have Rocs in my bag, but in heavy wind, I'd add a lightly used Champion Roc or a Ching Roc... in addition to my normal neutral Ontario and slightly overstable Super Rocs.
I always have Firebirds in my bag, but in heavy wind, I'd add a fresh max weight Firebird. Super windy, I might even add a Gateway Speed Demon. These are used more as mids than as drivers. Few things are more predictable in the wind than hyzer upshots with a Firebird.
In any wind, I'll always add a DX Pegasus I use as a midrange disc. Straighter than a Firebird and holds amazingly well into the wind. Flight path of a DX Pegasus upwind is almost identical to the flight of a seasoned Roc in no wind.
FAIRWAY DRIVERS:
I always have Teebirds in the bag. Windy day means I add a fresh 11x Teebird to the bag in addition to the beat 11x's I usually have.
DISTANCE DRIVERS:
I always have destroyers in the bag. Windy day means add a fresh Champion Destroyer to the bag... really windy day means add an Xcalibur...