I think the difference lies in that:
- threads do not permanently change or damage a route / rock face, unlike pegs
- threads (cord ones at least!) -generally- have much greater longevity than most in-situ wires, cams, pegs
- most threads (cord ones at least!) are easy to remove (and replace) unlike rotten in-situ wires, cams, pegs. So if someone wants to do a route without a thread they can (have it) remove(d) it; and if a subsequent person wants the thread back they can put it back in. No sweat, no damage done.
I agree long threads are offensive and are -essentially- cheating. And if a thread is not particularly hard to place on lead, best not to leave it in-situ.
I think in-situ tape/sling threads in sunny locations are a problem. But less than pegs.
Finally, we all need to remember that rock-climbing is an entirely pointless and arbitrary endeavour at the best of times.
