Here is an older thread that discussed this question. For me at least, it's not that single portafilter pull bad espresso, it's more like I can pull good singles, but using the same grind setting my doubles don't come out right, and vice versa.
Wait, how about a clarification here. Are you asking about a portafilter handle that has either a single or a double spout, or are you asking about using either a single or a double portafilter basket? I took your question to mean the former, rather than the later.
I've heard one coffee pro complain about the single spout not making good espresso, which as far as I am concerned is ludicrous, and makes absolutely no sense. Personaly I use no spout, as my portafilter has the bottom chopped off of it. It makes better espresso than either a single or a double spout handle, with more crema, and a better mouthfeel.
If you were talking about a single vs. a double basket, that is a totally different story. It isn't easy to get a great shot with a single basket. There are people who can, and do, on a regular basis, but it requires more work. Flow dynamics through the puck are much different in a single basket compared to a double, so it only makes sense that you have to completly vary your technique when using one. Doubles just make more sense. They are easier, and the results are more consistant.