Let me preface by saying that I dont think a secondary is necessary in the majority of beers.
A secondary is used to transfer beer from the primary in order to get it off the trub and stop fermentation...in my opinion a secondary is needed for aging beers longer than 3 weeks or for racking fruit beers away from the fruit.
Here is why I dont use a secondary....in the primary the beer goes thru the fermentation (in general) over a two period, after that the yeast continue to work (although very slowly) consuming heavier sugars and byproducts left over from the fermentation process...so racking at this point would prevent this from happening.
As for new brewers I would never recommend a secondary, because there in-experience at this stage of the process could allow them to completely ruin there beer...in addition I cant tell you how many topics I have read were a new brewers beer did not finish fermenting...so the advice I always give is leave it for 3 weeks!