Sure, a 9wt. two hander will handle a 20" brown with ease, but it will still put a good bend in your rod. But, it's not all about the fight and for myself, it's mostly about enjoying the Spey fishing/casting experience. I need to be able to cast too where the fish are holding and control the fly presentation as well...I know of many fishing situations/opportunities on the Bow where long casts are needed to reach prime lies that hold large trout...It's tough to reach those lies and control a drift properly without the use of a longer, more powerfull rod.
Not all of the trout in the Bow will be holding along the edges of the shorelines, as is commonly believed, but in fact on many days, the majority of the fish in the river will be holding along the edges of the main Thalwag...waaaayyyy out there...
Personally, I love fishing these mid river lies, from the shoreline, just for the challenge.
Here's a pic of a nice Broon that I caught in January 08', the first time that I used my new
Loop 14' 9wt...I had caught a glimpse of him rising out in the chop... It took a 100'+ cast to reach this fish...
A few swing presentations later....Wham...what a great fish, a great memory and a sweet rod too.