Shoe wear

When does the wear on the sole of a running shoe severely compromise its cushioning?

I land very much laterally (but roll in, so I'm not a Supernator) and I think scuff a little on my right side, which wears quicker than other areas. On my older shoes the sole's outer (4mm?) different coloured rubber grip layer part (if you know what I mean?) has worn off completely where I land.

Can't you tell the difference with the older and newer shoes when you run in them?

 If the old ones are as comfortable as the newer ones, they are fine.

 (Unless the newer ones aren't actually that new, and both have worn out!)

They are pretty much as comfy as each other really, but wasn't sure if that was where the buck stopped.

It's the mid-sole that really provides the cushioning, so you can't actually "see" if it's gone.
If you've got "cushioned" shoes, you should be able to feel some noticeable give, if you press with your fingers on the centre of the insole. If there's minimal give, then probably they're not giving you much cushioning. Trouble is, it's more of an art than a science, but worth comparing a relatively new pair with an old pair to see if there is any difference. (It will vary between models and brands tho, just to confuse things!)
There's a kind of rule of thumb as well tho, if you're not getting any niggles (knees / ankles etc) they may still be good enough for whatever running you're doing.