So, I was curious about this shield-wall debate and decided to take tests into my hands (using War Of The West). I added shield wall to the Hebridean Nobles and the spearmen. I allowed each to be charged by English Familiars, the best cavalry in the game. Here's what I've found;
Uachdaren vs. Familiers:
Shield wall - 17 lost, around 1 lost per charge (except for mistake I made, 10 killed)
No shield wall - 63 killed, 25 in first initial charge
Sleighdeirs vs. Familiers:
Shield Wall - 94 lost, many killed on first charge
No shield wall - 40 killed on initial charge, 50 killed on second. All 127 men lost.
Conclusion: Shield wall DOES in fact help defend from cavalry charges, MUCH more than without it (clearly). When one uses the shield wall, they must be careful to watch for flanking maneuvers - the cavalry often attempted this. Also, if the spearmen are engaged with cavalry, they need to be supplemented with some sort of flanking maneuver - whether by heavy infantry or a light cavalry. Also, if using multiple units of spearmen, it is best to array an especially large shield wall, by interlocking each unit together.
V - cavalry
= - shield wall unit
V V V V V V V V V V
= = = = = = = = = = = =