In today’s question, SDStaff Doug answered the the question but I felt his answer lacked the completeness I had come to expect from the Straight Dope.
The following link answers the question a little better in that it adds some info that I felt should have been included.
http://arachnophiliac.com/burrow/whydoesnt.htm
I thought Doug’s answer marvelled in it’s simplicity. All that needed to be said was said, and everything that was redundant was, well, left out.
puk, those were my second thoughts also. However, those other things aren’t redundant, it’s extra information. The facts stated should include that some spiders are built with hooks on their legs and that they have a special oil to help keep them from sticking to their web. It’s looking for and receiving extra information like this that gives scientists something to go “Aha!, I wonder if…” and who knows what inventions, discoveries or whatever will come of such investigations. My point still stands that Doug’s answer was not complete. It was only mostly complete 
and that means it wasn’t all the way complete.
Actually, sadly, the answer was not only incomplete, it was unsatisfactory as well.
It fails to address the issue of spiders who use other methods besides unsticky strands to navigate the web. It also fails to address why it is a spider can pull its foot off, but an insect can’t pull itself from off the web. The latter is really not hard to answer, but would help someone puzzling over the situation to understand the physics of how the web works.
Most importantly, if that answer was all that was needed, it was a waste of space better used for such wonderful answers as the origin of the Taos hum, etc. 