To what end? The only audience - other than engineers - such a network would appeal to is recruiters looking for engineers, which at least in the long-term would cause pretty much the same problems you described. What's the purpose of engineers only networking among each other? Engineering is not a bubble that lives independently of other professions.

Another point is: Would you pay for such a service? The main reason why LinkedIn is as it is, is that nobody (well, nobody save for recruiters) wants to pay for their service. Hence, they had to find other avenues of monetization and started selling their users' profile data. XING - a European competitor - isn't entirely free but had a fairly successful premium offering right from the start. From what I'm seeing they don't have a prevalent spam problem, at least not to the extent LinkedIn has (LinkedIn spam probably isn't even seen as a problem by the company at all but rather encouraged, they just don't say so openly).