Zebraligt is using Pogo Pins because of the high current demand in the new models - several Pogo Pins have a lower resistance than a spring. The 700er Flashlights even have Beryllium Copper springs in the pins - better electrical conductivity. The fact that Pogo Pins require an exact length of the battery is one of the requirements for an unprotected cell - protected ones vary too much in length. Two more reasons: Unprotected ones usually allow higher currents, and because unprotected ones are shorter, the flashlight itself may be shorter as well. Due to the high quality of the Zebralight circuit incl. internal protection, I do not see a requirement for an extra protection in the cell. Of course, this is only true because Zebras are single cell flashlights - a setup with several cells in series would always require protected cells.
Regarding reliability, I haven't heard about pin issues in the Zebralights except from the very first Pogo models.
The Acebeam 21700 is 74.8 mm long, I assume your source has just measured the base cell and made a mistake there. Look at the data sheet: http://acebeam.com/download/sample/11102
Thanks for the correction, great answer, thanks, I really appreciate it.
I'm just a bit wary of having unprotected cells around the house, in multiple devices, in chargers, etc. Just paranoid I guess.
I've had no problems with it with my Manker U11 (18650 based) flashlight so far. I was hoping for something 21700 based and not too much larger. I'm often biking or tracking my black dog at night (or both) and often have my flashlight in the 500-1000 lumen range.