Spare Tube:
- Some flats are not patchable - since I switched to flat-resistant tires the far fewer flats I get usually aren't patchable. For example if you get a flat right where the tube and the tube valve stem come together, and you simply cannot patch that.
- So if you want to be self sufficient, you *have* to carry a spare tube.

Patch Kit:
- Sometimes you change the tube and find out that you thought you got the thing that caused the puncture out, but you didn't, and you end up with another flat.
- Sometimes you legitimately get a 2nd flat on the ride (it's weird how they come in groups even if they're unrelated).
- Sometimes that other guy you're biking with has a flat, and you don't want to give him your tube and not have a tube, but you also don't want to insist he call for a ride or something.
- And the bottom line is that the quick apply patch kits are the size of the postage stamp, like 1mm tall or so, and cost $5 - once you're carrying a tube a patch kit is inconsequential in size.

Personally, I can't imagine not carrying both - I mean I'd be more likely to have a reason to carry neither than to have a reason not to carry both. :-)