Now I take my Canon dslr most places, but I tend to carry only one lens with me when out walking. Too much baggage otherwise. But I always wish I had the lens I left behind. Sod's law, and self-delusion, I suppose, along with the desire to do two things at once.
Yesterday I tried to catch a hovering tern with the telephoto. The result was, frankly, rubbish. I think it was a combination of lack of time/patience, probable need for a tripod, and not very good light. I'll post when/if I get a good result.