SPI have been using full GI pathtracing with Arnold renderer for the last 5 years, and Blue Sky (studio which did Ice Age) has their own GI raytracer they use as well.
Also, Pixar don't actually have that big a renderfarm - they don't need it. Other places like Weta and ILM have renderfarms that are much bigger, but are used for multiple productions at once, and for doing things like compositing and fluid/cloth/physics sims.
It's only recently that advances in processing power have made physically-based ray tracing practical for film production - particularly with the take-up of the Arnold renderer by various other studios. Suddenly lighting becomes a matter of placing lights and letting the computer do the work rather than needing to carefully set up the correct impression of light in the way a painter might. So it requires quite a bit of change of approach from the artist, and you can imagine why there'd be a bit of a cultural problem introducing this.
If I can make my webapp better (however you define "better") than my competitors' using PHP and MySQL, while they're making theirs using Ruby on Rails, MongoDB, etc,etc. Does the tech stack in the background matter, aside from making a nice article?
I think it does, because the tech stack in the background allows for things that might not be possible for other tech stacks.
You can duplicate somebody else's webapp in your backend of choice, but you can't have true GI if your rendering engine doesn't support it, and while you can fake some of the effects, they ultimately won't look as good as the real thing (unless you're aiming for a different 'good').
There's the obvious render time, but actually render time isn't that important - studios are happy to wait up to 30 hours for a 4k frame on the farm if that's what it takes for a shot. But they don't want artists waiting around, so they want very quick iterations and previews of what the artists are doing, as it's the artists who cost money.
This is why global illumination has taken off over the last 5/6 years (thanks largely to SPI and Bluesky showing it could be done), as although the render times are slower, it means lighting the scene (by phyiscally-based shading) is much quicker and you don't need as many hacks as you did with PRMan (light maps, shadow maps, reflection maps, point caches, etc). You can literally model scenes with lights as they are in the real world.
On top of this, there's how easy it is to do very complex shots and change just bits of it - tools like Katana allow hugely complex scenes to be managed and rendered very efficiently, with very little work from artists. Studios who don't have similar tools often duplicate and waste a lot of time doing things that should be easy.
For example, Weta on IronMan 3 wasted a lot of time doing all the different suits, as they didn't have a decent asset-based pipeline that would have allowed them to re-use a lot of shaders, assets for each suit.
So while I'd say they are still way ahead in efficacy they are at par or behind with respect to light simulation.