they are only there to stop crap from getting into the back of the disc

you can run normal sierra ones if you want to run new stuff (i've done it on the twinkly) but if you want to fit bigger discs you will need to bin them anyway as they get in the way

besides, there was an article way way back in some mag that said binning the dust sheilds made the brakes more effective as they let more air to the disc and let them cool down better