Ha! I'm such a long-time crocheter that I automatically default to fixing my knitting problems via crochet rather than finding a knitting solution! For example, when I was making animal hats, I tried knitting the circles for the eyes and found that to be a royal pain. Instead, I switched over to crochet and had circles in a matter of minutes.
I love crochet and think that knitters sometimes under-rate it, or even worse, relegate it to non-wearable items.
So far, I've used crochet for lace edgings, seams, reinforcements, as well as edgings that were only meant to straighten out a wonky looking cast on.
I also did one (just one) leg warmer that has knitted bands around the top and bottom and a crocheted design in between. If I ever get the other one finished, I'll post a picture (but don't hold your breath as these were for practical use--and obviously they're not really necessary these days).
I agree with others--knowing crochet (as well as tatting, hairpin lace, broomstick lace, Tunisian crochet, etc.) just gives you more options and fun things to incorporate into finished objects. What's not to like?