the yeasts are performing different duties during secondary fermentation. someone else with more knowledge can chime in and correct me if need be, but i believe during secondary, the yeasts clean up some of the earlier byproducts from the active primary fermentation, such as diacetyls as well as converting some fusel alcohols into esters. these secondary duties are operating alongside the primary duties as well since you will see that even in secondary fermentation, the SG will drop somewhat, but at a much slower rate till it plateus.
i personally use plastic for primary. i have a 6.5 gallon glass carboy, and i've only used it once. the reason being is that it won't fit well in my chest freezer that i use as my fermentation chamber. i would rather have consistent fermentation temps than be able to see what i'm fermenting.
like others have said, plastic buckets are much easier to deal with for primary, you are producing enough co2 to make any oxygen that permeated through just blow off through the airlock. plus, glass can be dangerous if you have an accident. there are a number of stories of people not only losing 5 gallons of beer, but also needing to go to the hospital and have multiple stitches.
