Well, these are fairly low melee accuracy values. Currently this stat is initially 20 - 60, which means that you need to be careful who you are equipping with melee weapons, and even the best ones are only so-so at the beginning.
On the other hand, some alien species are melee experts, so they employ these weapons quite effectively (or should, this is still largely untested).
To sum up, plasma weapons are not useless, but problematic. Otherwise they would be too good.