The ABS system on 95 M3s is a 4 detector (one at each wheel), 3 channel
activated system. The rear brakes are on a common line. Each of the fronts are
on their own line. This means that if the system detects lock-up on one of the
rear wheels it will cut back on brake pressure until that single wheel begins to
rotate again. Obviously this cuts down on the brake pressure being applied to
the opposite non-locked rear wheel as well. Hence your braking efficiency might
be slightly less. However, since the majority of your braking is done on the
front wheels the rear brake might only be seen under severe duty or extreme
circumstances with a significant difference between left and right traction (one
wheel in sand or snow, the other on asphalt perhaps?)
The 1996+ M3s have ABS that is 4 channel sensor, 4 channel brake line actuation
and so can vary the brake pressure independently on each wheel for presumably
better braking under extreme conditions or significant differences between left
and right traction surfaces to the rear wheel.
I personally don't see much of a problem with moderately different wear amounts
on rear or front. It becomes a problem if it is extreme leading to different
traction from side to side, or, if it is extreme you will be asking the LSD
(limited slip differential) to be doing more work than it usually does. I have
heard that if you are using a limited use spare on a LSD axle then you shouldn't
use it for a significant amount of time because the LSD (and its oil) may
significantly heat up potentially causing damage. I don't know if this is true
or not. But it does make sense.
And, in cars with traction control systems or systems that detect a
"differential" rotation between all 4 corners a significant difference in wear
of the tires may show up as activation of the traction control system or dynamic
stability system, or in the new E46M3, telling you that one tire is low on air
(the system works on the basis that a tire that is low on air will have a
smaller effective diameter and hence showing up as a different rotational speed
than the pumped up tires).
95M3CSL (no traction control, no DSC, no flats either.....just bald tires
happily worn evenly!).