Grey overflow into Black tank mod possible?

The easy solution is to keep the cap on and open both ports until they even out.  Not perfect since it will probably drip and a lot of water will be left behind the cap when you take it off.  Probably the cheapest of the better solutions is to drain gray water into a bucket and ferry it into the toilet.  Repeat a few times.

Nope you aren't the only one!  I investigated various methods last year because I was going to be on the road 3 nights in a row, and normally would fill up the gray tank after 2 nights with showers.  Since I could take more fresh water with me I wanted to use some of that black tank space.  Ended up using a campground on the 3rd night so I didn't need to do it but the bucket-to-toilet method probably would have been my choice.

This topic has made a question pop into my mind. 
My Rpod is the first RV I've owned that had grey and black water tanks, my prior RV experience was a Coleman Tent-trailer, which only had a fresh water tank. 
So my question is, why do RV's have separate grey and black water tanks?
When I go to dump them, they drain into the same disposal outlet, there aren't separate outlets for grey and black water,
in fact it's a bit of a hassle to switch the hose form black to grey, I get that it's nice to use the grey water to "clean out" your hose after flushing the the black tank,
but what would be the harm in them running together into one tank....?

I've used my FlowJet Mascerator Pump (aka "the poop cannon") to transfer grey water to the black tank via a hose into the toilet.  But, as the need to do this only arises 1-2 times per year, I haven't rigged up anything more sophisticated.

Ok, so elaborate, what are the advantages to having separate tanks?