You might want to consider it from this point of view:
There has been much said by the UK and EU about it being legally binding. If that proves not to be the case, it has implications throughout Europe for other countries.
I personally think that the document IS therefore legally binding, but even if it isn't, the political climate across Europe is such that, no one else wants to risk anyone else living and there will be a feeling of doing as much as possible to support agreements made to keep the UK staying in. Ultimately if they do renege on the deal, it will only destabilise the UK again, and the likelihood of a 2nd referendum on that basis will be high, since the result of the referendum is likely to be close even if Remain do win.
Other EU countries DO NOT want us to leave. Now or sometime in the future.
The UK is also not the only country which is unhappy with the EU and is looking for some sort of reform.
The political climate is sure that if any other EU nation does decide to not honour the agreement, there would be enormous fall out and pressure on them to change their mind: In other words everyone else will be on our side.
The countries most likely to do this, are the Eastern European members - who if you believe any of the stuff in the press, seem to be the ones with the most to loose. The ones least likely to do this are France and Germany, who see losing the UK from the EU as the thing most likely to damage them.
The balance of probability is stack strongly in our favour, imho. Legally binding or not.
FWIW, I think that if the UK referendum does vote to stay, the EU have been quite stunned by how close its been run and how much negative feeling it has unleashed. They do not want this in their own countries. I think the moment for more reform on our terms and to listen to the UK a bit more, is possibly more there than it has been. Not because they want to make concessions to the UK, but more because they have had their eyes opened to the possibility of similar ill feeling to the EU at home and questions about its influence and conduct. We are not the only country with national pride.