I got one but it was Class 8 not Class 10 like it is supposed to be. Also it is out of the way, not in or near my home System. 

I have gotten two free "lvl 10" planets.   One was lvl 14, the other was lvl 16, and both were in star systems adjacent to my home world.   I was actually thinking that maybe the idea was a minimum lvl 10, but evidently not!     They should change the language or make the planet always lvl 10.   And it should be "nearby", no doubt.

Personally the fact that they are not colonized is fine, giving a planet that is already at full population (and is not a captitol) would provide at least one difficult issue: its morale would be pretty low (at least for the couple of races I have tried out  --- the morale requirements have ramped up sharply in B5).   And imo, it would be a bit OP.    But that's just my take.

The biggest problem really is that the value of the planet is so dependent on the size of the galaxy and the density of habitable planets.   In a sparse galaxy it is huge, in a crowded galaxy it's not that big of a deal.

There is a destroy colony button in the planetary governors screen.

Ha, better and better, got the final stage of the top Bene branch of the tree, and *poof*, instantly, no planet appears.

Also, Darca, that's "destroy colony", not "scuttle the useless planet, and don't let anyone else have it".

I tend to play tall to avoid as much spammy-spam-spam as I can.

More than a dozen colonies late-game, and I find myself turning the game off out of constructor-spam-induced boredom.