Currently if you do not meet the requirements for a battlefield, you can't help anyone that does even if there is zero reward in it for you.

I was helping someone clear the final battlefield for San d'Oria last week and we needed help, but my friend couldn't help because she was from Windurst. She doesn't get any kind of reward for this, so why shouldn't she be allowed to help out? Just skip the Cut scene movie so there aren't any spoiler worries. We had to shout for 4 hours trying to find somebody before another friend logged in who was able to help*.

But this isn't just about nation missions. There are many areas of the game where this blocks people from progressing.

I hate it when I go to help someone and the game says I can't. That is actually what got me to clear all 3 nations ranks years ago. I'm just asking you to do is open up the help just a little more.

*Before anybody lols at our inability to duo it, remember not every job can do everything equally. Eventually a blue mage friend logged in...And probably could have soloed it with the damage he was getting from spamming AoE stuns...