Having toured through days of heavy rain with wet shoes, I can tell you that feet are very vulnerable to being continuously damp. In the UK we know the condition as "trenchfoot".
My waterproof Sealskinz socks were overwhelmed after a while.
Sandals are one possible solution but I don't like riding in them.
For everyday commuting in wet, cool, temperate conditions it becomes a real hassle to add an extra layer of waterproof bootie on top of your existing footwear, just to cycle to the shops or pub.