Bean, check out artifacts found in association with known megafauna kill sites.
Ok, let me try to simplify. If they had the atlatl available to them, don't you think they would use it to it's potential? It is able to fly further, penetrate deeper, it's reloadable, can be used at a greater distance..etc, etc. To say that they would use a thrusting spear in lieu would be similar to taking a knife to a gunfight.
After the quarry was dispatched, then knives and large blades would become a necessity. It's been shown that some smaller points in fact were used also as cutting implements, but flint cutting implements dull quickly and would need several resharpenings (thus a larger one that would last longer would be more appreciated). I have done alot of experimental archaeology, including butchering different animals with flint tools...learned quite a bit from the experiences.
Here's a sidenote, arrows sometimes also were used to "cut" with. I have found many that exhibit use wear and resharpening characteristic of have been used as a knife. Necessity is the mother of invention and whatever they had available to them at the time is what was used.