Having a well maintained LinkedIn page is a good idea if you're looking for work, or holding out the hope of getting better work.
Does someone from non-IT background (a teacher/ a doctor/ sales representative) get benefits in creating and maintaining LinkedIn profile? Yes. Teachers and doctors may not get much out of their efforts, but both positions are high in social interaction, so appropriate use of social media is persuasive. For a sales rep, this would be pretty much mandatory - if you can't put your best foot forward on a business social networking site just what is it you're selling?
How reliable is [the recommendation]? Does employer or interested parties take these seriously while evaluating a LinkedIn profile? This depends on the stature of the person(s) making the recommendation. If the person evaluating the recommendation knows the person making it, it's a lot stronger than if they don't. If the people making recommendations seem to have a weak relationship with the person they're recommending, then it looks like fluff. If the recommendation is from your boss of 10 years and details specific things he or she admires in your work, it counts a lot.
Is there any other constructive way I can use LinkedIn instead of searching JOBs and new opportunity? 'Instead of' - no. It all plays together. LinkedIn is one tool in the toolbox. If you don't use all you have you're simply discarding opportunities you should exploit.
Static content, such as a resume, has the potential to create confusion. If you customize a resume for an employer, and they find a different one on LinkedIn, they may try to 'mash them together' to see if they synchronize. If you have a number of short term gigs on the resume on LinkedIn, this might not be a good thing. Check off the skills you have and be conservative in your strengths. In short, if you think you're a 5 on C#, rank yourself as a 4. If someone sees your message postings in conjunction with your skills, they'll rank you according to their criteria in any case.