10% (load balancer downtime) + (.1 * .1 * .1 * .1 = .0001) = 10.0001% downtime. Not noticable over just the plain 10% of having just a web server. When you take into account the extra speed you get with the load balancer, plus the fact that load balancers are more reliable than web servers, it's a no brainer.
You got my point! Extra speed: yes. Extra reliability: Not really. Not if you add a single, non clustered load balancer.
And to strengthen it: Extra speed is what we need, here I agree completely with Lars.
Well, I got your point, but you didn't get mine. What I say above is that reliability with load balancers with be worse (on average) if, and only if, load balancers had the same reliability as web servers. They don't - they're more reliable than web servers, and thus, our reliability should go up.