Eric has written an interesting article about how your link portfolio can get devalued by the search engines. I must say that I agree partly with his point. Its hard to do anti competitive things to a big site, but for smaller site it might not be the case. But I feel that if you are doing link building correctly and keep getting quality inbound links the efforts from your competitors wouldn’t last that long as your good links will get picked up as well.
Furthermore those spammy links would just get devalued and not count in your overall link profile. Click here To Read more about when your link portfolio is devalued
To be honest, I think if a business owner who has the time and money to send spammy links to his competition should really evaluate his business objectives. Resource is limited, why not focus on something that will impact your site right?