We have the domain for business name, but we keep getting offered other more product specific names, of which we've bought a couple. Other than keeping them out of the hands of out competitors, I'm wondering if there's any point, as the domains have not got any relevent SEO history. I guess I just don't understand enough about this...
If you're being 'offered' domains, that's not because it's good for you necessarily. It's because they're probably in the business of selling you domain names. You're a good example - you've bought domains for no apparent good reason :).
Two reasons I would use these specific names. One is for type in traffic for specific terms or products. You can put up mini-sites for very narrow niches.
And the second is as you mentioned, it prevents others from getting the domains. I believe keyword rich domains are getting more valuable over the years.
You are getting taken by the oldest trick in the book. "It has been brought to our attention a company is attempting to register your domain in .cn .ru .dd .bb and so forth. Get the ones around your name .net .us. info and so forth and any country spacific you sell or do business with. The emails are a scam 99.99% of the time.