It's quite a faff to hide threads when the site is busy and it can take a while. I wish it were easier like on Yahoo. Each message has a 'box' next to it and you just click on the boxes for the e-mails you want to delete. Obviously we can't do that here but we could 'hide' them.
It's the Royal things that's triggered the question as the threads are relentless and posted for 'traffic', but I've often pondered this hiding lark and wished it were easier.