What's wrong with Hebbian synapse models?

[Hawley K Rising]

I am trying to get through a book on large scale neuronal theories (Koch 
and Davis, Large Scale Neuronal Theories of the Brain,1994 MIT Press).  
In the introduction they say that although they've had a pervasive effect 
on models, most theories about brain function don't last a decade.  An 
example they give is the Hebbian synapse (synapse strength grows in 
proportion to correlated activity of the pre and post synaptic neurons).  
What was wrong with this theory and can I get a source to read about 
whatever replaced it as acceptable?

Thanks in advance,

Hawley Rising
rising at crl.com