Be very careful with saying things like "no benefit", based on application memory consumption graphs.
More memory beyond those figures will give you more cache. If you run something else at the same time, or are using a hard drive, your application trying to use 16 or 24 GB will start causing your Mac to purge things from cache. Which means they need to be re-read from disk. Again, if you're on SSD this isn't such a big problem these days, but if you have a hard drive, or are using hard drives for bulk data storage, you will take a hit to IO performance. And even with SSD.... SSD reads in terms of gigabytes per second on new Macs. RAM (cache) access speed is hundreds of gigabytes per second or more.
More RAM is ALWAYS better (in terms of overall system throughput, not just one application), but how much better tails off beyond a certain point depending on how much data you're dealing with. It's obviously also more expensive. But given you can pick up 16 GB for under a hundred bucks these days, it just doesn't make sense to skimp on memory (i.e., run less than 8 GB or so) if your Mac has slots.
But yes, I agree - for most people, unless you have a very specific usage scenario, beyond 8-16 GB at the moment is probably overkill.
But if you're talking 4 GB - upgrading to 8 GB will give you essentially 3x as much RAM for apps. Because OS X itself and a small amount of cache will consume 2 GB pretty easily... leaving 2 GB out of a 4 GB machine left. an 8 GB machine will have 6 GB left... 3x as much...
