What
Please add "beta branches" of older versions of the game/tools to be able to be played through steam.
An option to play the game as "static", without updating.

Why
Mods
The experimental game/tools are nice to have to get mods updated,
because it's possible to run the reforger tools along side with experimental tools.

This helps updating things, because sometimes parts of my mod do not load up AT ALL in the newer version.
Audio files changing is a big headache and it's easier for me to open the old one and new one side by side.

Sometimes I don't have time to check out the experimental and update my mod before the main game updates.
I wish to be able to use 0.9.8 version of the game/tools to look at my mod before it broke with 0.9.9.

Legacy
Since arma features long-term modes, campaigns etc.
It would be very good to be able to keep playing
on a out-of-date version for an extra week or two, after the real update.

Imagine playing a slow paced, capture the island campaign with friends,
hopping on every night for a few hours.
A game update breaks the campaign and mods, the progress would be lost.

But using a branch of the game that doesn't update,
would alllow players to finish their campaign, even after the main game updates.

Updates
Having big updates for arma reforger is good and bad.
It's good to get changes and new features, improvements.
It's bad that big patches will break many mods.
With this ONE SIMPLE TRICK, we could enjoy both!