The problem with saying it in chat is the fact that there are many, many, way too many... BLM/SMN who feel like someone is actually talking to them.
I cant tell you how many times this has happened to me..
I do understand that you may want to help a player who is in the run for the first time, but seriously if the melee has 4+ soldiery pieces you can assume that he did it quite a few times.

There are so many things that can go wrong if someone writes in party chat to use LB. Caster can use it, Tank can use it (this really happened..) or worst of all you are in a party with an complete ignorant who wont LB just because you told him to use it.

I ignore it when someone tells me to use the LB.
Because lets face it: I. Know. It. Better.
We know how much dmg the LB does. We know when the right time for the LB is. Specially if the Melee is a Monk. Dont bother telling a Monk what to do. A real Monk knows the fight better than you do. We need to know it better in order to max our dmg.