First off, even when I lack sleep, I'll function better in the morning if I force myself out of bed, and I'll have a big slump in the afternoon.
If I sleep in instead to supposedly catch up sleep, I'll do OK.. but I won't feel any better in the afternoon.. I'll be a little off because of getting up late.
Secondly, afternoons are more depressing. I'd rather get out of bed, do a monday morning and then go home. It's really depressing in the winter, to leave work when it's already night outside. :/
But really screw "monday mornings" off, give me a monday off or nothing.
edit: not to mention the most obvious... people with long commutes coming to work for a few hours... really dumb.