No point in a referendum on the final outcome of the deal really. For one it would frustrate the EU beyond belief seeing the deal terfed out by the British public immediately, with two years of diplomatic capital flushed down the drain in that moment. And secondly, this won't be the last bit of diplomacy we ever conduct with the EU. Subsequent governments will continue to revise the relationship drawn up between the May administration and the EU for decades to come.
I'd rather have a 'soft Brexit' deal pass and amend it later than throw any deal out into the wind. Having some kind of formal relationship established post-Brexit creates a good rock to work off of.