What are the drawbacks of using synchronous ajax call?

Question

This question could surely be applied to jQuery but in this case I am referring to Prototype. In the Prototype doc it says,

Since synchronous usage is

Answer 1

Let's remind you that JavaScript is single-threaded

A synchronous IO call BLOCKS THE ENTIRE THREAD

A simple fix is to use asynchronous style programming using callbacks.

Customer = Class.create({     
    initialize: function(customerId, cb) { 
        new Ajax.Request('some-url', {
            method: 'get',
            parameters: {
                customerId: customerId
            },
            onSuccess: (function() {
                this.setCustomerInfo.apply(this, arguments);
                cb.apply(this, arguments);
            }).bind(this)
        }
    },
    setCustomerInfo: function(response) {
        //for the sake of this example I will leave out the JSON validation
        this.customerInfo = response.responseText.evalJSON();
    }   
});

var c = new Customer(1, function() {
     document.write(customer.customerInfo.firstName);
});