It really doesn't work. I've had lots of customers try it and called me in when it didn't work. I have never heard of it working correctly! DMX is not at all like audio. Opto-isolator splitters are not that expensive and solve your problem.
They also work when mixing fixture types (some don't play well with others), and when the strings of fixtures get larger. I've read that 32 fixtures is the limit, but I know of situations where a lot fewer fixtures didn't work until they were isolated/split.