Whilst even the biggest advocates of 'genuine lines' of earth dogs look at a terrier with bull blood and admire it's physique. After that, everything is going down hill work wise...
Those that breed them will say it's to strengthen snipey heads, or give them more fire, BUT you can improve all that, and retain working ability by using the right line of terrier.
All that said, if anyone is thinking of breeding such a cross, let me know, as I'll be after a new ratter soon, and I want one that's scared of the dark...