Its not a waste of time... Its a once in a lifetime opertunity to see eminem live in england when he hasnt done a tour in 12 years !!
I have owned every album of his, and find his music quite inspiring (the serious songs)
Lol he means it's a waste of time having to rush back up north straight after the gig.
The correct way to do it is book into a hotel, (Watford or Hemel Hempstead are handy for getting the train to Wembley) Arrive at lunchtime and check in freshen up etc. head out into London for a few afternoon beers, food, visit some sites or whatever. Head to the gig, leave gig and head back to some nightlife wherever your chosen hotel is. Sleep. Return home the next day with happy memories of a top day / night out in a London.