Humans can jump over barricades that are down, no problem.
Yes, down barricades are about as useless as any other non-functioning structure. That is if you're referring to when the OM is down and NONE of the Alien buildings are active. If you're talking about when an alien is standing beside or on top of the cade, causing it to lower, than all I have to say is...
"Wow, you or one of your teammates knew humans were coming (aliens have radar, you know) and you DIDN'T move away from the strategically placed barricade? Sorry but that's not the cade's fault."
As for the multiple-barricade scenario... What Moose said. Cades probably aren't your best bet there, which is when you use tubes/hives/trappers to delay/damage the enemy.
I'll give you a scenario where cades ARE effective:
A.T.C.S. - Alien Default - Hallway entrance
Not only does the cade fit perfectly in the doorway but it can be placed there instantly after the start of a game (meaning no need to build a 3rd egg across the way because the first 2 reach.) This is a great way to hold off at least part of their initial rush and ends up funneling most of the humans in through the front, which gives defending Aliens the advantage of attacking from above.
Basically, learning to use barricades is all trial and error. I suggest watching how other (effective) builders use cades and learn from their choice of placement and when they use them.