I took the number "90%" as an example. You can make it 99% if you like, the result remains the same: One loadbalancer in front of two webservers reduces availability. Simple maths.
I also think that load balancing a farm of web servers is the way to go, I just want to point out that availability will not increase.
This is not only wrong, but silly at the same time. Load balancers have better availability than web servers - if they didn't, nobody would even bother with them. They'd just have crazy schemes where web servers automatically take over for each other when others go down.
Plus, consider that when you add a load balancer in front of a farm of web servers, you no longer care about the availability of the web servers - because when a web server goes down, the load balancer takes care of it. So, by your 90% rule of availability, given one load balancer and four web servers:
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.