ive got the fiesta centre kit, i dont use or need the bias valves as mine is even braking, and i havent nor will i ever get a problem with the back over braking to the front.
ok ive got escort cos disc and calipers up the front, but mine being a FRST has different rear bias valves (factory fitted) to lower spec models so i wont say every fiestas will not need bias valves fitted.

1 problem/thing i didnt like is the fact the kid pushed the wheels out by upto 20mm a side towards the arch, but a problem i shall over come when my engineering workshop is set up, in the form of me making new hubs


although i have them on my rs, and it does look the nuts, rear disc on a fiesta is really just that, looks, the hand brake is far better on drums than disc, and the calipers are prone to seizing up if not used a lot or just gen a bit crappy to start with.
if you got money to burn go for it, if not, stay with drums, well, i say that, if its a 1.1 or something at least upgrade to 2i/frst drums