Bump.
I thought I'd try out the ooze shield on my Ultimaker S3 since one of my materials tends to ooze while the other material is printing. I can't understand what other purpose the ooze shield would have except to prime the extruder so that there's no extra material as it's just about to print. I guess I can understand how some printers may have dual extrusion nozzles at the same height, but that's not the case for the S3, so the "hopefully the material oozing out of the inactive extruder while wipe off on the ooze shield" approach is not very significant.
If the print order was reversed (i.e. switch extruder, print ooze shield with new extruder, then print layer with same extruder, switch and repeat), then the ooze shield would serve a dual purpose to both prime just before printing, and possibly catch hanging oozes.
That way users can choose prime tower OR ooze shield, and my preference would be the shield in most cases I've come across so far.
As an aside, it would also be very useful to allow prime tower or ooze shield for one material or the other, not tied to both, because some materials don't ooze so much and the priming may not be so necessary, while the other, dual extruder material might need it.