What's the point of IBUs if hoppier beers have lower IBU than stouts? I'm confused by the whole idea of IBUs. I see a lot of IPAs that are rated in the 40s or 50s for IBU, but in practice they are much more bitter than stouts in the 80+ range. Shouldn't the IPAs have a higher bitterness rating? What's the point of the rating?
IBU is a measurement of the chemical concentration of iso-alpha acids, not the perceived bitterness. Malty tastes do suppres the perception of hop bitterness. See wikipedia on beer bitterness 1 and its literature list. 

A heavily malty beer, which a stout is, requires way more chemical hop bitternes to achieve the same perceived bitterness as a light pilsner. 

Excuse me going anecdotal, but at one time, friends and me brewed an ale with an unholy amount of hops (150g @ 11% for 22l). When it came to hop additions, we decided to just dump in all the hop bags from the freezer. This ended up to be around 100 IBU after correction for ageing. 

Now, quite clearly, the resulting light ale was on the very edge of being consumable, but with some aging and getting used to it, it became a very very good ale, which all the hoppy people loved. 

The same amount of hops in an irish stout would probably be "meh".