I think the whole value of connections is diluted otherwise. A connection doesn't mean anything. So that's what I see as the disadvantage of adding as many contacts as possible.
However, if you're looking for talent, it is helpful to have such "connectors" in your network - without those recruiters, when I was doing a check on my network I didn't see some of the people I was trying to hire - this was only the case when I had switched jobs/domains.
I literally have 0 recruiters in my list of contacts.
It does however show the conflict again: recruiters are their bread and butter, so they may want to be careful outing them as people with huge weak networks that appear to be spam nets.
The problem is that the connection feature in LinkedIn covers 2 vastly different use case. What is regular for you ( and me ), a network of people we have worked with and developed a relationship and that overlap your competences in some fashion and eventually become an implicit reference for what you put in your resume. What is regular for recruiter is a network of customer to which you have provided services. You will look at a recruiter connection to double check if he is indeed in contact with all the major investment banks as he pretend, not to double check he indeed worked at company X on project Y in the past.
The weighting should be almost the opposite for a recruiter. If he has worked 5 year in HR in a company 10 years ago, those connections are less interesting that the one he has made in his current position.
Problem is that all the use case are treated the same in linked in and most people mix and match different type of contacts in their connection.
Most people never reply, so I presume they are just trying to bump up their numbers.
That's how connections who you do not personally know - help.