That's just my point...throwing styrofoam plates overboard really don't help the person in the water to any degree and in most conditions, unless you happen to have a C130 on call circling overhead, don't really help you find the MOB in the water.
The dinghy idea is a good one, but you'd need to have a drogue of some sort on the dinghy, since it, like the styrofoam plates, will blow down much faster than an MOB, due to the difference in the amount submerged.
It would help a lot more if you had a dinghy that was a bright color... most dinghies I've seen are grey, grey-blue, blue, or black... all of which are pretty good colors to blend into the ocean on a bad day. A bright yellow or orange dinghy would be ideal.
I'm not saying that saving the MOB isn't the priority... but I think that, we sailors as a community, really need to think ahead and avoid doing anything that is basically useless and destructive to the environment.
Most environmentalists understand the danger plastics do to all marine life. During all the years we've boated on the ocean and coastal waterways, I've gone out of my way to pick up any plastics we see floating on the surface. I even collect the debris I find while scubadiving. This is something every boater should be conscious of. However, when crew goes overboard - don't even stop to think about saving gulls and fishes, throw over anything that floats to save a person's life.
Off-topic a bit, but the various MOB deployments mentioned for both rescue and signaling, got me thinking about another possibility, not yet mentioned. I have yet to try it - never even had a MOB situation while underway, but it is another method which could assist the victim well during this time-critical situation.
Many cruising sailors, myself included, typically tow an inflatable dinghy astern. If a quick-release snap shackle was lashed to an aft cleat and secured to a tow line eye, the dinghy could be deployed by the helmsman within seconds of a MOB call. The dinghy would be very visible and also provide stable floatation until the mothership reciprocates.