May arguing that no deal would be better than a bad deal is just laughably incoherent.
Obviously all of Brexit is ridiculously incoherent, but it makes sense for her to take this line to strengthen the UK's weak bargaining position. In any negotiation the side that needs the deal the most is at a disadvantage as the other can walk away easier. Only the loopiest of fruitcakes actually want Hard Brexit but by positioning Hard Brexit as our starting position, it attempts to diffuse one of the EU's strongest hands which is that we need a deal more than them, by claiming that we don't actually need a deal and that we could just walk away. We obviously have no intention of actually walking away, but by putting doubt in the EU's mind we might be able to get a slightly less worse deal than would have been the case*.
Of course, most of May's bargaining isn't with the EU (who, incidentally, have already won the opening positioning battles by establishing that the negotiations will take place on their terms - e.g. only after Article 50 has been served) but with the Tory hard right, the handful of nutcase media barons who decide the red-tops' agenda and to a lesser extent the British public. And with the latter, it's useful to help position us to fear the worse so when we get a deal with some concessions we somehow think this is a success, even though it would still put us in a far worse position than we were in before the vote.
*The best way to achieve the ability to not need a new deal would be to put any final deal to the British people to accept the new terms or remain in the EU. Unfortunately Brexiters don't seem to believe in the strength of their, admittedly flimsy, arguments that we're going to get a better deal than we already have, otherwise they wouldn't be so frightened of letting the people decide in a second referendum between the new terms negotiated and remaining after all.
So May has to go to the House of Commons to trigger Article 50. This will dragggggggggggg this out for an age....but least if we have to come out it'll can be done correctly and benefit all of us rather than those who want to slit their own throat to spite their face.
The process will drag out for an age because it is hugely complicated to undo a lifetime's developments.
The decision doesn't mean it will be done correctly and benefit all of us, it merely gives us a better chance that it will. It's still highly unlikely that it will benefit all of us or even the majority of us.