I have one on my tandem and one on my beater bike. They have their merits.

Here's the downside. You've already made mention of all of the stuff that you carry with you. Bigger bag = more stuff, most of which is unnecessary. I have a trunk bag that velcros onto my tandem rack. We never leave home without it. What's in it? You're guess is as good as mine. I know that I have a couple of tubes and a CO2 inflator and a few tools. It always seems to be full of stuff that we think we might need so there's seldom room for things that we might want to buy along the way. Uh - it's been years since I've needed anything other than an inner tube and an inflator, maybe a candy bar from from out of that bag.

Incidentally, regarding bungee cords, I've got one that has flat straps. I think that it's a Blackburn product. You wouldn't think that flat straps would be very much different than round ones but they are. The straps don't seem to move around as much and they hold things much more securely than round bungees do.