Look at the Rogue trees, they're all about weapon types. One for stick builds that can be defensive or offensive with some nice synergy in Henshin Mystic for either playstyle, one for daggers where you can go for int based instant kills, or dex/str based and and go for more DPS. There's also a ranged tree for repeaters that does something I guess.
Point is a lot of the trees are weapon and/or fighting style based and having two of them dedicated to using a shield seems over the top, especially on the more generalist fighter class.