Duane,

I can't think of any reason that we'd need (or want) it on our groups.

But perhaps your members will.

One is strictly technical, so there's a remote possibility it would be
used there. One is a social group, so no need for it at all unless
there's a Dislike choice. A third is a planning/club group and I can't
think what we'd use it for there.

I think a reasonable theory is that many people chime in on lists simply to say "I like this", or "I agree with this", and no further comment is needed. The opposite, "dislike" or "disagree" typically comes with elaboration - why you like or agree with it - or begs for one. Having a dislike option might actually impede understanding if it causes members to simply flag their disagreement without explanation.

Of course there are lots of theories on this. See also:
http://www.quora.com/Why-is-there-no-Dislike-button-on-Facebook

If Like is the only option, should we assume that the other 7 billion
plus people in the world don't like it? Puts a different slant on
things. (grin)

A rather silly slant, if I may say so.

First, any assumption about people who did not see the message makes no sense. Second, you assume there's only two possibilities, like or dislike, where a quite natural possibility is that some or most people didn't have an opinion or didn't care.

-- Shal