LinkedIn is a social networking site for business professionals. In fact, it is heavily used by recruiters and employers to source candidates for employment. So, don't be surprised if you've seen people in Sales or Recruitment who are trying to connect with you.
In regards to your question about the positives and negatives of LinkedIn connections - let me provide you some:
1.) Career Networking - We can all benefit from building a network and LinkedIn can serve that role.
2.) Identification - LinkedIn is not going to work for you if you don’t identify yourself. However, if confidentiality is a concern, and if you are trying to stay anonymous, just be careful to connect with only people you know well.
3.) Search Engine Exposure - You can connect your blogs and other social networks to LinkedIn. Your LinkedIn statuses can be updated when posting to one of those places, providing the opportunity for greater network connections.
4.) Job Hunting - Let say that you are looking for a job, but you are currently employed and you don't want to jeopardize your current position. Be strategic and don’t announce to your connections that you are searching for a job. You can still do your job search confidential by not providing your company's email and phone number.