Towers isn't exactly heaving "off peak" - looking at today for example, the longest queue I've seen is 20 mins (good old Galactica) with everything else being 0 mins - 15 mins. Right now on the "Family" side of rides at Towers, 19 out of 21 rides have a 0 minute queue. That's a lot of staff doing not a lot - as said above, Merlin can probably cope with that better than a one off attraction could do.