On the M-types I can see no other function of the lips then preventing the outerrings from creeping out of the crankcase.
I guess it is a heritage from earlier models where the crank was not pulled up to a ballbearing but the lips were used to position the crank axially.
On the driveside the lip can actually lock up the crank if the 1" spacer is worn [like most of them] and the rollers are pushed hard up against the lip when the nut is fastened.
On the B-series where there is only a rollerbearing on the timingside it indeed prevents the bearing from creeping to the case. In the M-type the ballbearing prevents this.
I would say no need for lipped outerrings on the M20 unless your worried about creeping them out of the case.