As it stands currently, and it has gotten worse during "No-Proc", Ravenwatch and Blackreach are completely dead for 20 hours of the day.
There is around a 4 hour window (between 9pm CST and 1 am CST) where there is enough population to begin to actually play cyrodiil (aka: take keeps) on a map that has been gated by one faction for the past 20 hours.
So, if you don't happen to play in that 4 hour window than you are out of luck if the class you want to play happens to be faction locked out of the only campaign with people in it, which is ridiculous.
Unlocking Grey Host has other benefits as well, with the next biggest benefit being that faction balance will improve.
There is nothing worse in Cyrodiil than having your faction completely dominate a map. It is not fun for the factions getting gated and it is even less fun for the faction that is dominating the map. It makes Cyrodiil mind-numblingly boring.
Unlocking Grey Host would allow players to swap to a low pop faction and begin to pvp against the populated faction. This happened all the time prior to the faction lock.
Lastly, Grey Host needs to be a 7 day campaign.
Many are going to argue that "no one cares about the score" but that is just a complete misconception.
The score being "close" is what draws a lot of people into Cyrodiil. When a faction falls behind a great deal in the first week than the faction is almost non existent for the rest of the campaign.
Haderius, a 7 day campaign, was the greatest Cyrodiil campaign to ever exist and being a short campaign made it so.