It will require it to install, not to play. That's two different things.
Are you so sure... look at the pic below to see when Steam server become unavailable with a game who normally don't need Steam to play !!!
Well, since i am not really stupid and that i have the same game buy long time ago in retail, i have found a workaround for this game... when i have compare the medieval2.exe from Steam and on the CD, they was fully identical... where steam have mess up thing is with the launcher.exe ... so, the only way to play Medieval II without the need of Steam is to not click on it in steam client for start the game, not click on the windows menu or desktop icon... browse your directory and directly click the medieval2.exe ...
For history purpose, for people who have follow the Steam release of Diplomacy, one of the main problem was with the Sinslauncher too... and the advice was to directly start the game with one of the .exe ( sins or ent or dip exe) ... some of the comment was that Trinity was a crap game, where people are not able to play... no very good for the Stardock reputation...
Well, it is not important that there is a work around or not... the point is that sometime, game say to no need steam, when in fact need it at some point... For the case of Medieval II, once the game is started, you can kill the launcher.exe and close steam... it will continue to work... steam is needed for the launcher.exe and not for the game itself...
Point is that if player have similar screen that these above with Rebellion, they will be unhappy with Stardock... if a work around is possible, not all player read forum or wish to manipulate files for have their game working...
Almost forget, not sure that it will be used in Rebellion but Steam have a regional lock-up feature too... pretty nasty thing for people who travel a lot... by example, you buy your game in UK because you live in UK... for summer holiday, you go to US or Australia.... you try to start your game at your holiday location and it is no way... you can only play the game online in the area where you have buy the game...
Valve also in some cases restricts game registration and playability to the buyer's country of residence.
Not a big problem for a US customer outside in case of foreign holiday, or in case of military stationed in foreign country, or in case of business man who travel the world... but in my case, my country being so small that i am at a foot step ( a big one, around 50 km ) of numerous country like France, Holland, Germany, UK, and more, it is a other story... Steam is filled with plenty of nasty features that Stardock can choose to use... Customer ( pre-order ) and potential customer have the right to be correctly informed...