I think they would be very useful to keep pins off the bench and stop the board sliding around with every moved cable.
Lego works great for that: I screwed some medium-sized baseplates to blocks of wood (for mass and a little more rigidity). The spacing is pretty close to "right" for holding Arduinos. And if you have other, separate subassemblies (like pots or servos) you can easily build temporary structures for them that hold them steady relative to the Arduino.
I used to have an online photo gallery showing some examples, but the (bleep)s at Cox figured out they could improve their profit margin by .00001% by getting rid of their free customer websites, and I haven't found suitable replacement service.
The "snap-on" (or "stretch-on") holder would be cute, but it doesn't do anything for projects where you have other pieces that also need to be held.
Those thick plastic cutting boards are a good source of sturdy bases, too. You can get them for nearly nothing at yard sales, the plastic is easy to work with common tools, and you don't have to make an all-day trip to a plastics specialty store if you live in the hinterlands