The short answer is that mic cable will GENERALLY work in MOST small rig situations. However, if you're doing long dmx runs, using a lot of fixtures, splitters, etc., then you should be spending the money on proper data cable. The thing is that even tho a mic cable hasn't "failed" it might add in bad data, etc. due to improper shielding, etc. and cause lights somewhere in the rig to misbehave. The best reason to use twisted pair shielded cable for data is that tracking down a bad signal in your cabling is a tedious, time-consuming job and it's just more cost effective in the long run to get good cable than to spend the time trouble shooting. That time could better be spent with a pint of Guinness and a hockey game.
As for those who say mic cable is fine in larger lighting rigs, they are just lucky to have avoided failure yet.