Of course considering what a P.O.S. Sten mags are why would you choose to induce malfunctions?
Probably the biggest reason to use them is that they are cheap and fairly common.
While there were some shortcomings with the sten mag design, I have always felt that the common complaint about them being POS was a bit much. I use dozens of them (pinned to 5 rounds) with my sterling and they function just fine.
If the flat sides were dented, they could cause the rounds to hang up, and the angle at which the rounds were introduced had to be checked if the mag was giving you a problem, but I have found no more problems with sten mags than with any other mags out there.....maybe even less. The lips were always said to be weak, but they are double thickness, so I am not sure where that comes from.
The biggest detractor to these mags is that they need a loading tool to load more than a few rounds into, but since we are restricted to 5 rounds in Canada, even that it almost a moot point.
For shooting competitions where I use the sterling, I have shortened ex-cdn army deactivated (axe or hammer dent into the center of the mags) to a 5 round capacity. This way I can get lots of them into the pocket of my combat shirt, and I am not wasting space and weight carrying around 25 rounds of unused capacity.