1.) if you were allowed to keep equipped gear from the other faction when changing, this would demand that you max both factions to get the best of both worlds and keep changing back and forth to optimize gameplay, and would also require a lot of third party guides on what the strategies are regarding it.
2.) If you were allowed to swap very freely, but not keep equipped gear, this would result in needing to be specific factions for specific parts of leveling, again utilizing third party guides for strategies on this.
The idea for the factions is that you chose the faction for you - and you stick with it. Item Factions should not be related to a build, they should be completely based on your preferred method of item acquisition. Trying different builds shouldn’t be related here.
The cost is low enough, particularly early on, that you can try both at minimal cost. For when you have multiple characters, you should already know what each offers. If it’s your second character, and you try the “other” faction, and end up swapping away from it because you don’t like it - then that faction’s favour getting reset doesn’t really matter as you aren’t intending to use it.