I don't think this is the case. Admittedly, I have only used a shield for a year or so and a buckler for a few months, but I have watched it being used quite a bit and, even with my small exposure to it, it is pretty obvious that shields and bucklers are used with different techniques.
Granted, they are both used to block and parry but axes and maces are both used to strike and parry and we give them different rules too.
Historically, bucklers were *more* popular on the battlefield than shields both with knights and foot-soldiers for a variety of reasons. Personally, I suspect one of the main ones was cost. Shields needed to be replaced after pretty much any battle in which they saw conflict whereas the buckler (being made of metal) did not.
This is the first easy-to-read essay I found on Google, but there are others if you can be bothered looking.
In Mordheim their lack of use in general seems to come from the fact that all they do is grant a parry if you don't have one or a re-roll if you do, and parry is frankly weak.
In my group we are still looking at ways of making bucklers more interesting, but have not yet finalised on what we are going to do.
Things we are experimenting with:
Bucklers allow an additional parry if you have a weapon with parry.
Bucklers allow parrys on equal rolls
Bucklers allow an offhand that has one or more of the following:
The attack has concussion
The attack is -1 to hit
The attack is -1 S
The attack is +1AS
The attack is -1I
The attack is Always Strikes Last
We are also looking at adding shields skills and buckler skills that will do some of the above e.g. a skill that grants the offhand attack or extra parry or +1 to parry or something. Buckler skills will be in the Combat section and shield skills will be in the Strength section. Or maybe some will be in one or the other?
We are also looking at changing the base parry mechanic, both to increase the value of parry and the value of WS. Mechanics we are looking at so far are:
Parry on a flat 5+
Parry on a successful WS 'hit' against opponent (rolled separately and normal modifiers)
Parry on a successful 'wound', but with WS substituted for S and T.
Parry on a successful 'wound', but with WS+S substituted for S and T.
Parry on a successful 'wound', but with WS substituted for S and T, enemy S-mods apply.
As you can see that is a *lot* of combinations and permutations to test, but so far we are looking at giving the buckler an off-hand attack at -1S and granting parry or a parry re-roll if you already have a parry. We already have a house-rule that shields and bucklers add +1 AS in melee. The WS+S 'wound' gives the best numbers so far, but sacrifices simplicity more than we like so we are still looking.