I disagree. Atreus may feel very strongly about abortion being murder and realize that the two parties' candidates are very similar in other matters, bringing his vote to rest on that one issue. Zsdrsw for example is very much in line with my own beliefs, but is probably more or less a single issue voter because that single issue, gay rights, affects him more drastically (or at least more personally) than do the others. (Certainly I know other gays who are socially and fiscally conservative, but vote Democrat because they want to have the same rights - to marry whom they wish, to have an honest shot at a job or an apartment just like the straight guy - as do the rest of us.) Think about it - Obama is the farthest left President since FDR, while Bush is considered at least fairly right of center. How differently have they governed, really? The biggest difference is Obama seizing health care for the federal government, but Bush too had his huge health care entitlement program to increase the federal government's share of health care. You could take 95% of each's policies and most people could not tell you who did what. With the two parties so similar, it magnifies the effect where they do differ.
I'm by far not one and they tend to irritate me at times, but I think there are valid reasons to be a single issue voter.