Late to this thread - though I've read it through so will try not to repeat too much. I've ridden SPDs pretty much the whole time I've been riding MTBs - switched to clipless on a road bike back in '92, a few years before my first MTB so had already gone through the toppling sideways at the lights bit, and got SPDs very soon for offroad riding.
I've always primarily been a (XC) racer, hence SPDs make sense for the very real efficiency and comfort advantage. I agree that normally you're not pulling up, but sometimes it does help a lot to be able to. The rest of the time there is a physiological advantage to not having to put any work in to keeping your feet on the pedals - good flat riders might not have any trouble keeping their feet on the pedals, but that doesn't mean it's effortless. Also as mentioned above, there is a significant efficiency advantage to using stiff soled shoes which you can only do with SPDs - good for shorter distances, very important for longer where there is a huge advantage in terms of comfort and foot fatigue (especially for me now I have a chronic foot injury from running).
Suggesting that some DH riders using flats some of the time shows there isn't an efficiency advantage is spurious - the fact most of them do some of the time (still riding more gnarly stuff than most of us ever do) strongly implies there is an advantage. Some of them using flats just shows what we all know that there are some benefits to flats for that sort of riding - though clearly not enough to outweigh the advantages of SPDs for most.
Occasionally I have dabbled with using different pedals. Have used powerstraps a few times in events with lots of transitions between biking and running where it didn't make sense to change shoes - don't understand why they're not still popular, as I found them far better than old-fashioned toe clips and almost as good as using SPDs. Did some indoor races a few years ago, with even more transitions, but also where we were jumping on and off the bike all the time and flats made a lot more sense rather than toe clips or even power straps.
I have recently for the first time ever put proper pinned flat pedals on an MTB though - have got used to the idea from riding a uni and also now own proper flat pedals and shoes. I'm hoping to improve my riding technique a bit riding on flats occasionally - this is on my hardtail hack though, and I doubt I'll be getting rid of SPDs on my main full-sus any time soon. Really it just comes down to what you want to ride on though - advantages and disadvantages to both.
As for the OP though - and all those who have problems either with toppling sideways due to failing to unclip or with not being able to keep their feet on flat pedals - user error!
)
