From an seo point of view the benefit is having a link from different IP's so it's better to use different web 2.0s(IP's) for each keyword.If you create a decent web 2.0 and I mean decent links to authority first them your own it's worth the 20 crap ones you'll automate for 1 keyword anyway and won't get deleted.
You would then automate some web 2.0's to point to your good one.
It depends on the niche,low comp I'd do the quality web 2.0 everytime backed up by half decent ones pointing to it via automated software like ultimate demon.
Going further,and to me way more powerful,is creating satellite sites using aged domains.A satellite site is a purchased domain(pref by someone else) hosted by someone else so it costs to buy and host.These carry way more link juice done right than a web 2.0.
You could even try 301ing from an aged domain,search for 301's here there is some fantastic info on how to do it and right now they are crushing mega keyword terms like payday loans.