fuqnbastard

As Mr Mephisto noted, evolution doesn't happen for a purpose, it happens just because it can. The question "What is something good for?" is a question about a part of the integration of an organism in an ecological system.
It's gonna be hard to find an organism that isn't good for anything, if you count in what it's good for after its dead, because organisms will evolve that feed on it's body. Even virii are good for something besides replicating themselves, in that they kill people, which then can be soil for trees.

Lets construct a theoretical organism that is not good for anything.

(1) it must either not be mortal, or in the event of death destroy itself so completely on it's own that it's not viable for other organisms to either
- use the energy in their dead bodies
- capture the energy that gets free when they destroy themselves
It might seem like this is a gene that would have developed if it could have, because it prevents predators from hunting you. However, I believe, often that future generations are actually dependent on the dead bodies of their ancestors. Also I cannot imagine an easy mechanism to acomplish this.

(2) it must either
- not kill any organisms
- or destroy them as completely as described above

(3) the energy sources it feeds on,
- must not rely on being fed on,
- by the organism being fed on, no niche for other organisms may develop
- any energy used, must be used completely, no byproducts (i.e. excrements) may be a source of energy to another organism.

(4) it must still compete with other organisms to be the most efficient organism to fill the niche
For example: If it was more efficient not to shit reusable material, we wouldn't do it, but use the energy. Thus unlikely.

I believe I could go on, but I think the point is made: This is not something that would develop in an evolutionary process like there is on earth. There is no "useless" organism by that definition. Humans and virii come closest though. *sniff* ;]