Which numbers are you reading for this? The "memory use" total includes lots of irrelevant things like files read once during boot and optimistically cached for later, so you can expect it to be nearly 100% of installed memory at all times.
You're not in a memory pressure situation until you have an actual performance problem or the swap space starts growing.
But if you are having performance problems, I'm pretty sure someone cares deeply about them and would like to know the per-process memory use.
Once the shared libraries are in memory, memory usage grows really, really slowly for my application set (listed above) on OS X.
I'm not saying that 8 GB will last forever - I'm just saying, as a power user who has zero need for more than 8GB right now, that 8 GB on OS X will be just fine for about 95% of average consumer use. Absolutely would not recommend upgrading an iMac's memory unless you are really certain you are going to hang onto it for more than 5 years - not the best use of $200.
I'm just talking about your average family use; for vertical niches (Video Work, Graphics Design, Heavy Industrial Programs, Databases) - obviously those people will assess their situation and make a decision that is optimized for their circumstances.