Cybermen are half robot, half human.
During all the show, the Cybermen are eager to assimilate humans to increase their ranks.
But, in The Pandorica Opens, we see that a Cyberman, with its organic part long time dead, still appears completely functional. We know, too, that the personality and memories of the assimilated humans are wiped clean (except for a couple of cases).
So, Why are the Cybermen so eager to assimilate humans when they could just work as robots?
The amount of human and machine making up a cyberperson seems to vary form story to story. Here is a small exert from wikipedia that I've formatted:
It is presumed (and often implied) that there are still organic components beneath their suits, meaning they are actually cyborgs, not robots:
In The Tenth Planet, a Cyberman tells a group of humans that "our brains are just like yours",
- By the time of Attack of the Cybermen (1985), their brains seem to have been replaced with electronics.
- In this same story, two human slave-prisoners of the Cybermen on the planet Telos, named Bates and Stratton, reveal that their organic arms and legs have been removed by the Cybermen, and replaced by Cyber-substitutes.
In Earthshock (1982), the actors' chins were vaguely visible through a clear perspex area on the helmet to suggest some kind of organic matter.
- In The Tomb of the Cybermen (1967), Attack of the Cybermen (1985) and "The Age of Steel" (2006) veins and brains were visible through the domed head of the Cyberman Controller and similarly. The first cyber-controller is a Mondas Cyber Controller, while the second involves alternative Earth's John Lumic.
- In Revenge of the Cybermen (1975), the Doctor says they are "total machine creatures".
Clearly there is some conflict on how much of a human is left, however as a template the cybermen are mostly locked in as using humans, even if everything is later removed. This can be seen as a typical 'Kettle problem', where the logic goes as such: If you want to boil a kettle full of water, you fill the empty kettle and put it on to boil. If you start with a full kettle, you empty it and use the previous solution. Even if doing so is less efficient it's a tried and tested solution, that doesn't require extra work or creativity.
And as we've seen the Cybermen and also the Daleks (similar cybernetic/organic creatures) both suffer from a lack of imagination and creativity; lacking the ability to change. This is itself I believe a creative decision by the producers and writers I would imagine1.
The moral is, after losing their humanity (i.e. what makes them human, not just their morals) they are no longer able to adapt and change, regardless of how ruthless they are. No matter how much better a fully cybernetic cyberman would/could be, they cannot and will not accept that change. And no matter how much 'easier' it would be to simply build a fully cybernetic cyberman, they just don't have the creativity to do so.
Though it has been overturned, at least in a small way with the Cult of Skaro, but the mainstream daleks still obey this rule, and still make a good analogy with the cybermen
The purpose of a Cyberman's implants (or cyber-suit, or however you prefer to think of it), is to extend and enhance the life of a human ("Revenge" implies that the person's consciousness may be a sufficient preservation, if not their organic components). Much like the Borg, the Cybermen see this as a favour, not as an imposition - see Jackie's "I went first" speech in "Age of Steel".
The example from "Revenge" aside (which goes against all other established cyber-lore), an independent cyber suit doesn't seem to be able to think for itself, since it's nothing more than a life support system - the one in "The Pandorica Opens" is looking for another living human to "complete" it.
