If you look at the original STDP paper (Bi & Poo 1998) and any paper that measures postsynaptic potentials, it's pretty clear that synapses can be and often are weighted. They also come in a huge diversity of different sizes, and this has some relationship to the evoked synaptic potential. And this is true for pyramidal neurons too (Bi & Poo was hippocampal pyramids), it's just that we don't know whether or to what degree the weight matters, computationally.
You get a lot of power even just from binary weights as Numenta, Kanerva, etc have shown. So the weights may be a sort of epiphenomenon emerging from the need for a permanence value.
However, a great many computational neuroscience models of pretty much every part of the brain make a lot of use of graded synaptic weights, and they do appear to be present in the brain, so I'd be surprised if evolution didn't find a use for them.