If EZ opened a base at MAN, it is still under contract to retain its services from Liverpool until some year in the future (2017??), so it would probably open more niche routes from MAN than it would be able to from the smaller catchment area of LVL.
From MAN EZ would be more likely to open routes such as Riga, Ljubljana, Tallinn, Dortmund, Istanbul, Bilbao, Split etc, which do not have the same competition, either from existing MAN carriers, or to it LVL services.
Even if EZ did open routes from MAN to Prague, Alicante, Paris, Palma, Nice etc, I'm sure they would have the economies of scale and marketing advantages to compete profitably against the likes of Jet2, BMIBaby, BAConnect and Monarch...
Much as I love LVL, I think EZ would give a better range of destinations with greater frequency, if it focused on MAN instead of LVL! (+ Ryanair poses less of a threat at MAN)