The premise of voting the lesser evil is that the two parties actually compete for voters on the grounds of Hotelling's principle of minimum differentiation. The idea is that both competitors gravitate to the "center tendency" in the population they serve. Going for "lesser evil" makes sense because while both competitors are far removed from the extreme positions, they are not equidistant to those positions - i.e. one is somewhat closer to one position than the other. Therefore, voting for the one that is somewhat closer is more beneficial than voting for one that is somewhat more distant.
However, I think this premise is false. The two political parties do not compete for voters, but collude with each other to control government and sell political patronage to wealthy sponsors. To do this, they need to minimize voter influence instead of competing for voters. This pov is supported by studies cf. Walter Karp "Indispensable Enemies" or a recent Glies and Paige study.
This, of course does not mean that both parties do not compete for voters at all, but rather that they selectively disfranchise those segments of the population that are likely to challenge the party monopoly and its business of selling political patronage to those in a position to pay for it. It is not election in any meaningful sense but erecting Potemkin villages by busing supporters to create an illusion of democracy.
In this game of bi-partisan collusion, political patronage peddling, and voter influence minimization, the "lesser evil" argument is not a "median voter" choice but a stratagem used by both parties to keep voters in line and make them vote for each other instead seeking alternatives to the two party monopoly. It is not much different than two rival gangs demanding protection money on the threat that the other gang will do more damage if the ransom is not paid.
On this premise - voting lesser evil is not rational. In fact it is rather detrimental to voters' interests as it assures the continuation of the bipartisan protection racket that robs voters of not just political representation but their tax money as well. It is far more rational to take risks and go for alternatives to BOTH parties. It is more rational because voting for "lesser" or "greater" evil will almost certainly produce no benefits to the voters, while voting for an alternative have a small but real chance of ending this bipartisan monopoly and their protection racket.
