I'm more inclined to take a full spare outfit (rod&reel).... so for example, for a day's fishing on the wye, I'd take two avon rods, one quiver tipped and one hollow top tip, both rigged and ready with a reel holding 12lb line. Then a float rod with a centrepin, 5lb line.
On the thames, it would more likely be a standard feeder rod (8lb), the hollow top avon (12lb) and a float rod with a fixed spool (6lb). This would also likely be the same combination I'd take to a lake.
With these combinations, I could fish two rods if I really wanted to. Most likely i'll only fish one at a time, however for some reason re-tackling a rod with a different reel seems like way too much effort mid session (even if in reality that's only going to take 10 minutes!)
I have to say that using either rod ready sleeves or a quiver type holdall (I take both!) makes all this a lot easier. Typically I will have half an hour in the garage the night before I'm going to just to check that I've got everything rigged as I want it. Some of that stems from my own impatience, but as I'm also increasing taking my 9 year old son out with me, the quicker we actually get a bait in the water the better...