Let me try:
When you're bad at blocking here's what can happen:
-You CAN lift your shield all the time, save for your arm being broken, you're completely exhausted or there's not enough room to do so (in which case your skill wouldn't matter anyway), you CAN always block no MATTER of your skill.
-When you deflect a blow you can stagger or even be knocked down because you can't really balance out the blow well
-The shield can be knocked out of your hand since you lose grip
-Somebody can easily pull the shield out of the way and directly attack you
-A successful block drains a lot more of your strength since you can't counter it well
-A very strong blow can more easily injure your arm since it's "thrown around" and you can't absorb the blow too well putting too much stress on your arm
-You can't hold the shield up indefinitely, it will drain your arms strength, when your arm weakens you block also weakens
-It will take longer for you to "recover" after a blow (lift your shield back into blocking position)
So in the end you can't just keep holding up your shield and expect to block well but you CAN still block.
And what exactly do you see as "catastrophic failure" for a block?
EDIT:
I don't wanna get into that topic too much but there are better alternatives for the "failure" system, I wouldn't say "you have a 1/4 chance of success" but your opponent has a high chance to break your FIRST block and then connect into a second attack, that means you first BLOCK succeeded but it "failed" to hold. you won't magically get hit THROUGH your shield but you can't hold it long enough to deflect a second blow. Your skill pretty much determines how well you can CONTROL your shield, even IF it gets knocked around it might not be far enough to leave you OPEN for a attack, and that can be simulated by simple "physics", hell not even going so far, calculate the power of the blow, it's direction, your position and how well you could hold your shield, if the blow was strong enough your arm is pushed out of the way leaving you open for a attack, if your arm is too weakened you could drop your shield and so on.
The random factor is not entirely removed, I grant that is still a "necessary evil", but SEVERELY reduced to a size it won't be too irritating anymore and so many factors playing into it that it just feels more natural.
EDIT 2:
Oh yea and ONE reason against a automatic blocking system, sometimes it simply IS better not to block as it could throw you off, and if the game then decides to do it, that would svck.