They're a proxy yeah, but it's also somewhat loose.

There's a lot more going on at synaptic clefts, so just knowing the number of synapses doesn't tell you enough to model anything. You still need to know which neurotransmitters are used, how much, second-order effects like G proteins, basal firing rates, distance to the axon hillock, the shape of the neuron's effect on potential decay, etc.

And that's all to predict whether one neuron will fire. You could get very different behavior between different two neuronal pairs having the same synaptic count.