The other Morrigu variants are extremly rarely used. They have their purpose (in theory), but are never seen in battle, since the UAC10 Morrigu is just such a versatile death machine. Versatile in the sense of: it provides decent range, high DPS, lots of armor and GECM to stay somewhat undetected until in range and thus is the simplest weapon of choice for most situations.
However, it's not the variant that needs change imho, it's the UAC10 that is a bit too powerful in general.