Networks are more expensive to upgrade and limited by physics...especially wireless systems e.g. Cell and satellite. Smart caches can help a lot here, but not if everything is ssl.
I think the security gained from adding SSL far outweighs the efficiencies lost by losing a shared network cache.
That's not a security sensitive situation - in cases where security is at issue, the best practice is to make secure pages as lightweight as possible so they'll transfer quickly on slow lines.