Never heard about "Decoupling" or "Modularity" or "Reusability" or "Interoperability"?
SCADA is about collecting and processing data and/or supervising technical equipment, MES is about manufactoring planning and effectiveness. Why binding them toghether should be better? Single machines usually do a fixed (or more often a parameters driven) job for their whole lifetime, but this is not true for the plant, where optimal behaviour, KPIs and production can change from season to season. MES, in our modest opinion, are in the domain of "industrial ITs", not in that of plant engineers: colored meters and bars, or plant areas maps in both interfaces should not be mistaken belonging the same family.
Of course you can push a SCADA being a low-level MES, or a full fledged MES being a sufficient SCADA, but those are two different worlds "by design". Data structures are different, data aggregation is different, calculations are on far different time domains: why you should ask a SCADA to keep track of year-by year deltas on highly processed variables so far from machinery?
In year 2012 the "silo" paradigm has already been deserted by almost everyone. Distribution and specialization is the winner... for the moment... but, of course, it depends also on the target industry.
And you? What do you think?