The problem I see with offline testing (as in, spawn in unlimited resources and ships) is a large bulk of gameplay would exist in those offline environments as everyone realizes it is more efficient to test there than in the live game. It's just faster. As such, we lose the possibility of running into random abandoned test ships, or ships that didn't work out in combat as the designer hoped being left as wrecks among the asteroids - so many small events that actually add up into tremendous amounts of gameplay would be lost. The problem of "how to move asteroids" becomes quickly solved with no action in the live game, eliminating an untold amount of gameplay and interaction that would occur if these things had to be tested in the live environment. Every interaction left to the offline environment devalues the main game where those interactions could have taken place in the presence of other players who would appreciate simply seeing someone solving problems, moving around a station, checking the market for a material they want to test and so on. All that bulk content is eliminated when everyone retreats to an offline world to do their testing, eventually getting bored of testing and perfecting their designs after blowing their own designs up 1000 times, and then loads into the main game, to find it empty of players, no one to actually show your design to, because everyone is in the offline mode.
If the devs thought the offline mode was good enough, they wouldn't have decided to make it an MMO component. They already know - the game isn't the same without people. They want people. And I think it is important they find their path and decide which it will be - a mixed online/offline Space Engineers or a next generation MMO. The game flourishes with more people, not less, with more interactions, not less, and to make the call to have players only able to experience this game with others would go a long way in solidifying the game's future, by allowing it to have roots and a history, fully built by organic interaction on the live server.