What about PowerPoint's AutoSave?
If you've visited PowerPoint's Options dialog, you may have seen a Save option that says something like "Save AutoRecover information every XX minutes" where XX is a box where you can choose a number.
Beneath it are a couple other related options.
Note that it says AutoRECOVER, not AutoSave. It's easy to assume that this is an AutoSave feature, one that will cause PowerPoint to save your work every so often, but that's not what it does.
Instead, it saves crash recovery information periodically. IF PowerPoint crashes and IF it's smart enough to realize "OH! I'm crashing!", it can restart itself and recover your presentation up to the point of the last AutoRecover save. That's a good thing, no question.
But AutoRecover saves to a format all its own, not to a PowerPoint file that you can simply open. If you do something unfortunate by mistake or if a crash is severe enough that PowerPoint can't catch itself, the AutoRecover and its data won't help you.
For example, suppose you've worked on a presentation for three days nonstop. You tell PowerPoint to Quit and mistakenly click NO when it asks if you want to save. Sorry, you've just lost three days of your life. All that work is gone. AutoRecover (not AutoSave, remember) won't get it back.
How can you protect yourself?
One simple strategy: Give your files names that include the date.
Whenever you're about to make any substantial changes, do a File | Save As and give the file a new name: MyFile_2014-07-30.PPTX or even MyFile_2014-07-30-001.PPTX if you're making lots of changes in July 30th.
If you'd like something a bit more automatic, try PowerPoint MVP Shyam Pillai's free Sequential Save add-in. Configure it and then just click the Sequential Save button to have it automatically save your presentation under a new sequential name.
Either way, you'll always be able to work backwards to a previous file that was working correctly, no matter what's gone wrong in the current version.