Is it ok to draft or not?

There’s probably a few reasons not to:

• You’re relying on the guy in front not to make a mistake and send you both into a pothole or the back of a car.
• If the commute is really a contra le monte type deal (and let’s face it, it always is) then you shouldn’t really be drafting, should you? (You should be wearing a skin suit and an aero helmet and grimacing like Jan Ulrich after a wrap of speed in a nightclub and too many pies, but that’s another story)
• It’s a bit rude to suck somebody else’s wheel.

The last one is the one I’m not sure about. Is there an etiquette to urban riding, whereby you should swap duties on the front? After all, there’s nothing more annoying than somebody riding on your wheel and never making any contribution. On the other hand, I’ve got a riser bar on the front of my bike, so I’m never going to be as aerodynamic as the chap with drops, and if they overtake you, well, they were going fast anyway, so should they really complain if you tag along? Take last night; I was pretty knackered, bumbling along between Brixton station and Herne Hill, and somebody goes flying past me on a Scott carbon. I draft him until I bottle it when he goes between a van and a taxi, and then gradually reel him in, until we get to the lights at Herne Hill. Not that we exchange any conversation, and I was encountering competency deficiencies with my SPDs that evening, but on the way from Herne Hill up to Dulwich, the joy of a mountain bike block means I could still take the front, so there was some exchange of effort. Then he smoked me on the first bit of Crystal Palace, but since his home was only a quarter of the way up, I guess he didn’t need to save any effort. So you nod to the other guy as one of you goes past, and that’s that. It’s not like we’re looking to have a discussion of gear ratios, after all.
[Then again, if you wanted to go all Eddy Merckx on it, the sole reason for drafting somebody is to destroy them, and all you’re doing is saving a bit of energy for later use against them. But possibly that is quite rude]
And after all, by drafting them you’re not helping much, but (if my physics isn’t too out of line) there could be some imperceptible benefit to the man in front. Oh, and I have to ride over Crystal Palace, which most of the people I draft don’t seem to, so maybe I need all the help I can get.