not patching your tubes is wasteful on a number of levels.

1) waste of money
2) waste of resources (where that tube going when you throw it out?)
3) waste of skills

on the commute, i replace the tube with a good one and patch the old one at home later. unless i also puncture that tube, that is.

it's good to keep the patching skills tuned-up -- you can carry a lot more patches than tubes.

i generally keep patching tubes until the valve fails or it gets a tear bigger than a patch.