I should add another type of voter represented by the focus issue model: the sine qua non voter, for whom all possible candidates must score over a certain threshold on the focus issue but, beyond that, being better on the issue is of relatively slight importance. This is indicated by an equivalent candidate line more nearly horizontal than one for a dominant-but-not-determining focal issue voter. (The solid line starting at the dashed threshold line T, vs. the dotted line running through p, q, r, and s.)
Note that this is a descriptive model. It describes how people actually vote, or at least how they say they plan to vote; it doesn't prescribe how anyone ought to vote. (Not that people plot the candidates on a 2-dimensional color-coded grid, but they do say things like, "He may be better on domestic issues, but he's pro-abortion," which can be modeled as q vs. r.)
So, what can you do with this model? Give qualitative descriptions of candidates, for one thing. "Are you crazy? He's lower left!" "Yes, he's over my threshold, but he's down at the bottom." "Sorry, but given the nature of politics, I think a high-upper left candidate is better than a middle-right candidate."
It can also help explain and examine voting strategies.
There is no shortage of socially conservative American Catholics who say things like, "I can't in good conscience vote for a pro-abortion Democrat for president." This makes them threshold voters, refusing to vote for candidates below a certain level of pro-life policy.
What's interesting is how many of them define their threshold to lie between the "best" Democratic candidate and George W. Bush. Granted, there's a lot of space between the best Democratic candidate and Bush on life issues, but I suspect for many their threshold is highly influenced by practical considerations.
Not that there's anything wrong with that.
Still, I've been struck by the thought that some absolutist single-issue voters, who in effect say they must vote for the most pro-life candidate, would if they're serious have to vote for Jcecil3 if his name got on their ballots.