There is no reason for them to be bad, unless it goes against a specific snake's needs. For example, rubbermaids have a tendency to stay very humid and the ventilation is low - even if you melt/drill holes in the sides. So a snake needing moderate to low humidity would probably not do as well in a rubbermaid as in a tank or cage. But a snake with high humidity needs would do well.
They do have many good points:
Opaque sides: Rubbermaids are usually a little see-through or not at all, which makes a shy snake feel much safer.
Light: Rubbermaids are LIGHT. I can easily pick up a 50gal rubbermaid with the snake and stuff inside. Try doing htat with a 50gal glass tank, even without the stuff!
Inexpensive: Rubbermaids are cheap, depending size and brand, and can range from 3$ to, say, 30$ (though I'm sure it can go higher), which makes them great for litters or clutches of babies. It wouldn't make much sense to buy 20, 5 gallon tanks for 20 leopard eggs that are due to hatch and be sold.
That's all I can think of now, but as I said it depends on the snake/reptile's needs. Obviously a chameleon would NOT do well in a rubbermaid, but a rainbow boa would.