^ Nothing's been rebooted, though. As the new 'top gun' in charge of the overall production of the X franchise, Bryan Singer chose to help Matthew Vaughn create a story in First Class that disregarded the events of Origins: Wolverine; it didn't suddenly create a new continuity, but simply chose to behave as if the events of Origins: Wolverine didn't happen. This is also being done by The Wolverine, I believe.
The Last Stand is by no means a bad movie, and can very easily fit with First Class and the first two X-Men films. However, if Bryan Singer - who, as noted, is now the person nominally in charge of the X franchise as a producer - chooses to render the events of that movie as being out-of-continuity with the rest of the franchise using the events of Days of Future Past, it is within his perogative, power, and purview to do so.
I liked The Last stand myself, but the primary reason I think Singer and Vaughn ought to render its events out-of-continuity and pull a 'Superman Returns' with Days of Future Past is so that, in the event Singer opts to direct another X-Men film (which could very well happen at some point) and said film is a sequel to his first two films, he is not beholden to events that happened in a film in which he had no involvement.