I dont see anything worng with it, they may not be uk based so no use to them and it serves them right for not buying it themselves.
Yes its common for bought domains not being used, I have another business and currently have 6 domains purely so no one else gets them lol
Me too - I wanted to register the .com of my website name (which is a .co.uk one) and was furious when I found it had been bought already by some firm in China - when I emailed to find out why, and what they intended doing with it I got a reply that said it was soon to be used for a clothing line (and to be fair, the guy sounded a bit surprised when he'd looked at my site

). After this I bought both the .com and the .co.uk of my full work name justin case, and I suspect lots of people do it.
It's not expensive and as Friday says, even if you do nothing but buy the domains and forget all about it, it stops other people using them.