I think and voted that there should be a clear warning that you will not generate a block for years.
I still think that not even mentioning block generation is much better, if the option will stay. It should be like running Seti@Home, purely to support the network.
Though as stonetz said, it will become pointless at some point, if it isn't already, so maybe we can advertise some easy to use GUI GPU miner in the download pages or the client itself, so that the clueless user can run the miner without any hassle together with the client (then server mode would need to be on by default or be able to be configured through GUI).
It's already pretty much pointless. There's no sense in deluding newcomers into thinking that by taking the effort to waste power on this they are supporting the network in a non-negligible way. Humans' incentive structure is weird, if you tell them something will "contribute" they won't bother to ask "how much does it contribute?".
On the other hand, when a person contributes something insignificant (or believes he does) it causes him to be more responsive to making substantial contributions later on. This is an argument in favor of including the "feel good" button, but not strong enough in my opinion.