While I agree with practicing at tempo well below one's own threshold does nothing little to help gaining speed, the real question is what is that threshold and where to play around that threshold.
In my limited experience, the speed thresholds is when I begin to make occasional mistakes. If I begin to make same mistake in repeated runs, or I feel I am losing control, then that's above the threshold - time to stop and reduce the tempo! Even at the threshold, where occasional mistake happens at random slip of mind/finger, I still find prioritizing accuracy (by a couple clicks down on my metronome) is much more effective. When practicing just below my threshold, accurately and in control, for a little while, I can usually raise the tempo back up a few clicks and achieve the new speed still accurately and in control. Rinse and repeat that, is what I find the most effective way to raise speed.
In addition, scales & arpeggios are different because there mistakes generally means wrong or missed note. When practicing pieces, occasion mistakes does not mean just wrong notes. Even after I can play through a piece at tempo, I have often had to slow down to get articulation, dynamics and pedaling right before adding speed back again.