If you've a replaceable mech hanger (or steel frame), probably not worth bothering on an mtb either. One alternative - but meant more to save the frame/hanger than the mech is a replacement for the driveside QR nut, which reaches around (ooer) into the inside of the mech bolt to support it. I've put one on my '03 Spesh Enduro 'cos the dropout design is absolute pants - the frame bends before the hanger does
. Spesh fit (or used to fit) their own version to their newer freeridier Enduros, and on-one do a cheap knock-off version too..mine's the original CNC'd Hanger Banger, the on-one version's arguable better with bolts for sram or shimano and a bit more reach inside the bolt
http://www.on-one.co.uk/i/q/FSOOBH/on_one_banger_hanger(hmm, reduced to 2.99 and out of stock...mebbe discontinued..?)
A little more of a faff getting yer back wheel out, still doesn't protect your mech tho'.