I've made several rough projects where, just for practice, I used pocket screws, including a couple sawhorses, a little outside bench for taking off shoes, and my own workbench. Each time, I actually just used regular drywall screws instead of the Kreg screws, for convenience and (a very little bit of) money saving -- not because they're at all the right screws.
One thing I really had to do with the drywall screws that I wouldn't have had to do with the right screws was to pull the joint very tightly together before screwing. This is because the pressure on the screw to start tapping wants to push the joint apart -- even with a real Kreg screw. But then, the threads of a drywall screw run all the way to the head, so the threads near the head grab the first board, and don't let it pull snug against the second board. This is why Kreg screws have the partially unthreaded shank; that unthreaded part slips right through the first board, so the joint pulls snug.
In summary, for a rough project like your workbench, you can use any screw that fits if you're willing to clamp the joint very tightly before screwing. And while the self-tapping feature and wide washer on Kreg screws are very nice features, the one feature I would really look for if I couldn't find Kreg screws is that partially unthreaded shank.