It strikes me as a bad idea. I could possibly see it being okay if one leg was in fact noticeably longer, but IMHO the vast majority of people are better off with matching crank arms. If it's a bike you plan to ride a lot, this may lead to joint and/or back problems in the long term. No I'm not a doctor, I only play one on the internet. It also begs the question: What's the proper saddle height for mismatched cranks? Either 2.5mm too high for one leg, 2.5mm too low for the other, or +/-1.25mm off for both legs. If someone raised my saddle 2.5mm, I truly believe I would notice and would want it lowered before the end of a long ride. I know it's not much, but why not try to get as close to the right fit as possible? I think there is an unnecessary risk of developing problems from the cumulative effects of added pelvis rotation caused by using different crank lengths.

Btw, what type of crank? Maybe someone has an extra arm to make a match.