What are they for? I've never seen anyone with anything bolted to them, so what use are they?

More to the point – can I cut mine off? If I unship my chain (And we all do it however well the mech is set) the cahin goes behind the tabs and jams itself. Can I just cut the tabs off, and file them smooth?

Alternatively is there a plate or similar that I could bolt to the holes to stop the chain falling down behind it?

What are they for?

Attaching chain devices to.

You can probably cut them off, on most frames they are just tack welded onto the BB shell. Or you could fit one of these…

Which is designed to catch a dropped chain, it also has a bash guard integrated to protect the oute ring.

Can I fit a chain device, a bit simpler than the one in the piccie, and keep my granny ring? The thought is get a simple device, cut off the wheel bit and use what remains as a means of stopping the chain getting caught behind the tabs.

it would be a sure fire way no ruin any re-sale value your frame may have if you hack saw them off, plus you can gaurantee it will void any warranty you have. leave em on, they probably only weigh a few grams!

get a simple device, cut off the wheel bit and use what remains as a means of stopping the chain getting caught behind the tabs

That would be fine, backplates for chaindevices are reasonably cheap, just have a look on CRC for the cheapest part that fits and hack it up. If you match the upper profile to be snug against the mech (like the pic above) then it should work quite well. Then again your chain shouldnt be jumping off anyway, mine hasnt for quite a while.