are they even worth colonizing?
They are due to the fact that low class planets can most of the time be terraformed much more than planets with a higher starting class.
For instance a class 1 planet may end up beeing a class 16 planet after you have researched all terraforming techgnologies.
A planet starting at class 7 on the other hand, will never be terraformed into anything better than a class 10.
There has been some debate regarding wheter or not this is a good thing for the game. Some people thinks this is a nice touch that adds depth to the strategy of the game.
Others, like me, thinks that class 1 planets are just supposed to be crap-planets and more or less should stay that way or at least not turn into paradise-planets.
If you do not colonize but couquer a low-class planet, however, then it works differently since in that case, for some reason, the planet cannot be terraformed as much as all. (and it has nothing to do with the chosen invading strategy as the phenomena persist even would you flip a planet culturally)