Malwarebytes is an on demand malware removal tool, whereas security essentials is always running and checking things on the computer. You typically only need to run malwarebytes when you suspect there's an issue and security essentials is always running and should prevent most issues from ever getting on your system....not necessarily ALL, but most.
From time to time, we have been tempted to believe that society has become too complex to be managed by self-rule, that government by an elite group is superior to government for, by, and of the people. But if no one among us is capable of governing himself, then who among us has the capacity to govern someone else?
-Ronald Reagan, 1981 Inaugural Address-