It seemed like a good idea to have the OSR, 3E/ Pathfinder and 4E players sharing the same table. WOTC probably needs it to happen. 5E faces a very strong headwind of players that look at their current game(s) of choice and say, "I'm good." Some of these public playtests could get chippy, RPGers often use civility as a dump stat.
This is how I look at it-
While that has been discussed, I think more the expectation is that each table will play it's own style. I really think the whole inclusiveness and modules and such for Next is for that reason.
They let loose the Genie with the OGL. Then moved to 4th when many players were not ready. So up came OSR games and Pathfinder. I really think what Wizards is looking for, as a customer base, is the core that play the current edition of D&D because it's the current edition of D&D, then grab a percentage of players of those others (4E, Pathfinders, OSR games) then they grow their consumer base. This much is pretty obvious from a business standpoint. That is the only way for them to keep D&D alive - it has to find more customers - players/GMs that buy their product.
That is what Hasbro wants, and WoTC is trying to do.
With that as the basis - they designers know the business aims of the product of D&D - now they, the game people, not the business people look for a way to do that. The modularity, dials and switches make the game adjustable to exactly the kind of play a group may want. Difficult goal, but I think it is achievable.
So 4 tables may be playing D&D - and while the characters could play in each others game, the actual play at the table would be vastly different - one doesn't really use skills much, or do much customization, and is all Theater of the Mind, another is very player choice oriented and grids all the combat with a tactical focus.
Technically the same game, but the end result play is vastly different.
With the ability to create the game experience you want at each table there is a greater chance for greater sales.
Great that is what the business side wants - but what does it bring to players?
- Some may find that D&DN is better and bringing the old feel than the current game they play - they are already playing a game but it may not be exactly what they want.
- Some may want the faster supplement train that WotC usually have than the slower rate than they may have.
- Adventures may be more plentiful (I hope on that one).
- Perhaps a player plays two games (as people that play in a 4E and a Pathfinder games) - this would give them one base game with the same very basic ruleset but different gameplay will make it easier to learn and play.
- Similar to above, if a group decides to play a new campaign with a different feel, they can add/subtract modules moves dials and switches and play - without learning a new base ruleset.
- Some people like to tinker and adjust things and want to play with those options.
- The modular nature allows the Game Designers to come up with really cool options - that because the base system is designed for the modularity - make it much easier to design for and add into play. This could allow for 3.5/Essentials ect without the need to invalidate anything that came before.
To be honest, as late as it is, this may be difficult for them to pull off - Pathfinder is really set in it's place, and the OSR games have got passionate adherence. If they had announced this when the Essentials came out, and the next two years of 4E was the lame duck era, rather than starting now - a lot of those OSR games would not have the traction with players they do. It will be a tougher sell.
Much like Dancy's network externalities and the OGL, the modularity and "1 game to rule them" has 2 big purpsoes - make the game that is being produced the one that most people play, and the came can be made better with simple add ins to rules (an opportunity lost by WotC by not adapting any good 3rd party OGL stuff).
tl;dr - Wizards and Hasbro is trying to recapture players playing D&Desque games with a game that can recreate different feels at different tables. So yes it needs to be the 1 game to rule them all.
