I think that's true up to a point. It's mostly true if a,b,c,d and e are approximately equal, and the harm from the options not supported is within a certain range. But it's truest when most of the issues are trivial or temporary, or morally insignificant. In reality, there are likely more than three issues per candidate, and not all issues are equal. To pick a reductio ad absurdum, if one candidate says "I'll do this nice thing and that nice thing, but I support abortion," your opposition to abortion might well tip the balance, but that stand might not be enough to motivate a vote for someone who says "I'm against abortion, and I also intend to murder my opponents and commit genocide." Supporting any candidate is, if not supporting, at least discounting, the issues outside your chosen one, even if you pretend it is not.
You might well feel, and might well be right in feeling, that the current instance is one in which such dire choices do not apply, and others may well be wrong to believe, as some do, that it is. But the idea itself is not so trivial or so easily dismissed. It's no sin to want autobahns or trains that run on time, but we ought to have learned by now that sometimes the price is too high.