One of the better abilities of the Malconvoker is Fiendish Legion. It's huge, you get twice as much power per spell. However, if you didn't lose caster levels, you could use your higher slot spell to summon 1d3 creatures from the lower level, which averages to 2, or you could summon a more powerful (or versatile) creature available only to the higher level list that might be the perfect tool for the job. If you lose 2 caster levels, you're just as good at summoning as a regular caster, but you're worse at the rest.