Black on Maroon has been carefully restored by a team of experts having been defaced in October 2012 at the Tate Modern. Several advanced restoration techniques were used in order to ensure that this valuable piece was not ruined by the actions of one individual, so that the rest of us can continue to enjoy the painting in London.
Rothko himself left several canvases to art institutions in London and was a big fan of the city, making a successful restoration of this painting particularly important. Black on Maroon holds value, both monetary plus also through importance to the modern arts scene of the 20th century more generally.
You can read more on the restoration in this Guardian Rothko article.