Well, maybe not ALL of them, but go read
this article. My boss sent it to us, highlighting the importance of the development of the brain's executive functioning (self regulating one's behavior).
If you don't have time to read the whole thing, it's basically outlining how all the stimulating, educating, developmentally appropriate, (I'll add expensive and sometimes annoying) toys that we give our children can sometimes hinder this self regulation. When children play with no toys at all and are left to imagine, they use private speech to narrate their actions. This is the part that aids in the development of executive function.
The most poignant part of the article was about an experiment done with 3, 5, and 7 year-olds sixty years ago. When asked to stand still for a certain amount of time, the 3 year-olds were not able to at all, the 5 year-olds could do it for about 3 minutes, and the 7 year-olds could for just about as long as they were asked. Sadly, 60 years later, the results were grim:
"Today's 5-year-olds were acting at the level of 3-year-olds 60 years ago, and today's 7-year-olds were barely approaching the level of a 5-year-old 60 years ago," Bodrova explains.
Toys have their place, but they should not always be the main object of play - a child's imagination is a bottomless toy box.
*** Update: I still think baby toys, such as jumper-oos, are INDISPENSABLE ... but you catch my drift :) ***