Weighted pressure plates outputs a signal when a player or mob stands on it.

Intended behavior since snapshot 13w36a
Weighted pressure plates can now tell you how many entities are standing on it.

For Light Weighted Pressure Plates, every entity adds 1 strength to the signal
For Heavy Weighted Pressure Plates, starting at 1, every 10 entities adds 1 strength to the signal

Cool

So you can for example weigh how many mobs are standing on that pressure plate? Ok, interesting ... but with MC-29778 you still can't properly weigh items.

I'm not sure about this "intended" bit. It isn't in the update notes in either Mojang.com or the MineCraft wiki. And, the wiki on the Weighted Pressure Plates article makes NO mention of this feature.

Again: The wiki is no official feature list, it can be wrong. Also, is there anything bad with being able to weigh entities? I could think of some interesting uses for this ...

I knew I had seen a legitimate something-or-other about it.
http://imgur.com/fIbYNVB
I think there's more, but this is what I found first. I'm positive this is intended, I just can't remember where I saw it...

Thank you. That certainly is a lot better than a mod war between "Yeah it is," and "No it isn't."

It's still missing from the patch notes in both the official Mojang page and the wiki (in both update notes and the pressure plate page). So, I'd assume a lot of people don't know about it. Perhaps it's a WIP or someone forgot to right it down in a snapshot. Should probably be done.