I'll undercut by 1 gil because of two reasons:

(1) It really really pisses some people off so there is always the possibility I get a good laugh out of someone else's rage. You will then see these people get pissed off and cut by massive amounts out of spite. I'll then buy these and then mark EVERYTHING up to prices well above where they were before so LOL jokes on you.

(2) To me it shows respect to the other people crafting/gathering and selling on the MB. Keeping the price cut low keeps the prices high thus we're all making more money in the end.