I see the lack of necessity for these devices on road bikes, but what about mountain bikes? Are they acceptable on the trail?

Not if you properly adjust your derailleurs. I used to own a hardtail and rode the heck out of it up and down the gnarliest trails I could find. I never had a dork disc and never found myself needing one.

Depends if you believe you'll unknowingly get a bent derailleur hanger. A slightly obscure problem, but not impossible.

Sounds like the dork-disk needs to go before I further compromise my reputation!

The inward/outward movement of the derailler is controlled by the derailler cable, not the chain. The chain breaking wouldn't cause the derailler to move in or out.

Oh...and before I learned how to work on my own bike, I had an old diamonback mtb with a dork disk. There were tabs that held it to the spokes and once they broke off the disk started rattling. My solution? Zip ties! I actually drilled three holes through the disk and used little zip ties to re-attach it to the spokes! Doh!

Riding with mud on your back and legs is more dorky. Not to mention uncomfortable.

Yes, several of those, especially the 4 ft flag, are dorky, but as aMull said, riding with mud on your self and clothes is worse. And I believe wheel reflectors are mandatory by law in my province.

I live in southern California. What's this mud thing you are talking about???

don't they have mudslides in california?

What happened to me (on a new bike with less than 500mi) is one side of the chain broke causing a single link to make a V shape which the caught/jam in the derailler and pulled it toward the wheel.

That's what I meant by 'partly break' a chain. A full brake would cause no harm.

Al

I learned a new word today.

You forgot to mention a 2 pound kick stand

I recently explained to my friend that it's okay if he goes ahead and removes the reflectors from the wheels of his Cannondale. I also suggested the large bar-mounted bag needs to go if he truly wants to be aerodynamic, but to the contrary, he explained how he puts an extra pair of socks (and underwear?) in that bag because it gets too sweaty. I hesitantly inquired if he was using lycra cycling socks or conventional cotton gym socks, and I believe his response indicated the lattter....

I think you'll find that there is a spring in the derailleur that places it under tension toward the smallest cogs. Thus, an increase in shifter cable tension will work against the spring and shift the derailleur inwards, whilst a decrease in tension will do the opposite.

This also explains why shimano STI shifters have the downshifts on the brake lever - to get more leverage against the derailleur spring tension.