The slightest amount of contamination, eg. frost on a wing similar to a very fine grade sandpaper, results in up to a 30% reduction in lift and a 40% increase in drag. Add to that a failed engine and the bravest out there might start to sweat just a little.
On which wing would that be? A DC-9 I presume? But what about the Pa-28, DHC-6, ATR or perhaps a sail plane? It all depends on the type of wing - especially wing loading and engine location. A thin layer of rime on my DHC8-100 is no problem - even with an engine failure. A DC-9, on the other hand, requires de-icing almost all year round.
And so I agree with punkalouver. You have to know your aircraft. We've all seen the NASA videos from the Twotter tests - but those were ridiculous amounts of ice, and I cannot believe that anyone would be foolish enough to attempt a take-off with that **** stuck to the wings/tail. I still think there's a huge difference between, say, the F28-4000 accident at La Guardia, where the wings were FULL of ice and snow (and they rotated too early), and a turbo prop with low wing loading and a little residual ice or frost.