The shape of the tip of the bristles is critical for a good job. Cheap brushes are cut off square as in the second photo, which makes them easier to manufacturer. But they don't do nearly as good a job of carrying paint or laying it down as a brush with a tapered end and split bristles as in the third photo above. The paint stripe on the left, with all of its streaks is from a single pass with a cheap brush. The stripe on the right is one pass with a split end tapered brush that is carrying more paint and laying it down smoother. The rigid edge of a cheap brush actually acts a bit like a scraper as only a few bristles are in contact with the surface. The tapered brush has many bristles in contact with the surface at the same time and the split ends help the paint to flow. It is simply amazing that with the very same paint, the quality of the brush will make a total difference in both the ease of painting and the quality of the finished job.
Is there a role for the disposable foam brushes? I use them for two things. Using any paint, a quick touch up is usually not bad with the foam brush. Where the foam brush actually works better than other brushes is with clear finishes, but there is a catch. If you dip the foam into the finish, let it soak up some finish, lift it out without scraping it on the side of the container and flow it gently onto a surface without squeezing the foam at all, you get a wonderful smooth clear finish as in the left of the last photo. If you squeeze the foam into the container creating air bubbles, let it drip back into the container creating more air bubbles, scrape it off on the side creating a bit of foam on the top of the container full of finish, and then firmly wipe it onto the surface squeezing air bubbles out of the foam, you get that rough bubbly finish as in the right hand side of the last photo. Properly used, foam brushes can be the easiest way to get a smooth clear finish.