Have I tried it before?
I have tried this solution to the problem you are presenting here a few times in the past. I've tried it with 3 of my groups at the time (about 4-5 years ago) with different amounts of success. I've also seen it used in 2 of the groups I've played with, when I've still mostly played, and again with different amounts of success.
Is it fair?
I think that it is mainly group specific. Some of the groups I had really liked the idea, while others practically hated it. The real answer here is not on the web but in the minds of your GM and the other players. I would really ask them, politely as I can, but still ask them about how they see it and from there I'll decide.
What Issues can you run to?
I think that first and foremost, if not the whole group is for this idea it can really hurt the sense of unity in the group. It is hard to plan something with a player, and then when another one plays her character to plan the thing all over again because the new player doesn't know what has been planned or this isn't her style or something.
More than that, people can feel alienated from the group. Group dynamic is something that takes time to build, and each and every time that you add players you need to start building those dynamics from scratch. This means that more time of every session will be spent in creating those dynamics again. More than that, because some of the players will play the same characters in different times, it will feel even more alienated when the group dynamics will start to form between the characters also. If with one character the dynamic is more of a master and a subordinate when it is player A and BFF when it is player B the dynamic with both of those players will take a hit and something will feel stuck.
More than that, some players have preferences for which characters they want to play. When you give them what is free for them for this session you might give to a player a character type that she doesn't like. This can result in a bad game experience for that particular player and she may not come again (or at least not with the same enthusiasm).
What are the advantages of such a set-up?
First and foremost, it frees the GM from the need to play those PCs. We don't want the GM to have GMPCs, and this is the main danger when we give PCs to the GM. After all, when we give her those PCs, we actually give her the excuse to have GMPCs.
More than that, we free other players from having to play those PCs, thus letting them play their characters for the fullest experience possible. This means a greater chance for great roleplaying.
In addition, it adds voices and brains to the group when some players (brains) are missing. This may help to solve riddles or to persuade NPCs, for example. It also gives all of them more time to think about things.
My present solution
As I've stated in the first section, I don't use this solution anymore. Nowadays, when I have this problem, I play in the "Guild-of-Adventurers" format. This means that whoever comes to the session gets the time to play, and every player has a character of his or her own. This prevents most of the damages without sacrificing that much of the unity of the campaign.
The main thing to keep in mind, when using this solution, is that adventurers should be quite episodic. The adventure should end when the session ends.
And an end
Hope I succeeded with helping you a little bit.