For both brands to have amp simulation is nothing new, Marshall has the JVM and Mesa has the Mark IV and V that attempt to simulate analog-ly (the word does not exist but what the heck) their older models but with some of thier own new flavours. The JVM's cleans are something really spectacular.
Mesa does things the analog route by using valves and components to get it's simulations while Marshall uses software (like Line 6's POD) to get the simulation.
The route that Marshall is taking is more bold, for the fact they are a company which got where they are now via their use of valves. To me, if Marshall thinks that the product was not good enough, they would not have put it into production.