Not everyone on my friend's list is in a LS I'm in, or in my FC.. nor am I always in a party with them. Additionally, not everything I see needs to be in a forum where everyone can see it. If I'm giving a party member suggestions, I would much rather do it through whispers than call them out in the middle of a party. If I'm whispering a friend, I don't want to have to wait until I'm out of a dungeon to continue the conversation. If I have a comment to make to another FC/LS member that I maybe don't want to be public knowledge, I don't want to have to wait until after a run.
I totally understand not wanting to get tells while in a raid.. but if it's that big of a deal wouldn't it be better to have a chat channel that lacks whispers in it rather than take out option to whisper for everyone in the game? Seems very foolish and over-protective to disable the option at all for the "well just in case this happens" scenarios. I would like to think the average person playing is smart enough to setup things in a way that suits their playstyle best.