Think of the possibilities! Spiders would be so useful if they could be trained.
Obviously the first use that comes to mind for trained spiders is to have them make webs across your windows to keep out bugs. You might need more than one spider, depending on how big your window is. But really, everyone wins. Spider gets bugs, you don't. And unlike most window screens, completely organic! It wouldn't even matter that it would break every time you threw your cat through it, because the spiders would just build it again. Happily! They kind of expect it anyways.
The next best use for trained spiders is to have jumping spiders patrolling the kitchen. I wouldn't have to worry if I left food out, because the spiders would eat any bugs that got in. They could also patrol around my bed. If I lived in a basement they could just patrol the entire place. So long as I could train them to stay OFF the bed. I wonder if a spray bottle would work like with cats.
Then there's the spider silk. Man, get some spiders working on that, and you'd have super strong string, which would be awesome for reed making! Not to mention the million other things we use string for, like venetian blinds and garrotes. You could even teach them a dance so they could weave fabrics. Think of it! 6000 thread count spider silk sheets! Actually that's kind of creepy. Never mind.
But things like uber-strong cables for bridges and space shuttles and stuff would be useful. Engineering, military applications, who knows! It would make so many things possible that I can't even imagine.
Of course, then it would make lots of money and there would be people breeding spiders specifically to make the silk and they would probably live in horrible conditions and just work in tiny spider sweatshops all the time with nobody to stand up for their rights, until some protesters decided to take up the cause and have demonstrations at companies' doors all covered in spiders that would be trained to jump on executives and weave really strong webs across the doors so nobody could get in to work then when public awareness was raised they could take it to parliament to get a spiders' rights' bill passed or something but there would still be all those protesters with attack spiders out there attacking people and someone would get the idea of using poisonous ones and you would have to be really careful walking down the street and IS THAT THE KIND OF WORLD WE WANT TO LIVE IN??
Not really. Now I'm kind of glad it's not possible.
Yes, I'm sober.