It's generally bad. It harms players that want a friendly, but competitive game who don't mind disclosing their armies to their opponents while turning good generals into lazy generals who don't have to manage thinking on the fly.
I play with my father every now and then and he loves to ask me a week ahead of time what army I'm using or, if we're playing BattleTech, which 'Mechs I'll be bringing over. I don't mind telling him because...well...he's my dad. Besides, I'll usually give away this info to others that I play.
What ends up happening is he tailors his 40k or BattleTech force to oppose my army and stops there. I usually end up winning because I haven't spent a week convincing myself that my army will do the job for me.