and for the Scottish council elections, where you vote with numbers and can mark off as many as you want, there has been much debate on whether you should rank all the candidates, or only the ones you like. It has been argued that putting a candidate last ends up ranking them lower than leaving it blank, but I forget why, and whether this is actually the case or not. Confusing!
That STV system is a sensible one, but I can understand why people might think it's a stupid one, or get it confused with the stupid one used for the Scottish Parliament.
(My view is that the Parliament has a stupid system that guarantees seats for idiots because the people with the power to fix it are the people that think they might actually want a nice guaranteed seat, thanks. Whereas council seats are beneath them, so okay, you can have a proper democratic election system for those.)
Anyway, it doesn't matter at all whether you explicitly fill in your last preference. In any count - at every stage your vote is counted for your highest remaining unelected preference. It can never be counted for your last preference, as there must be at least two people still in the race for it to be counted, and you will have ranked another person higher. So "single blank" and "lowest number" achieve exactly the same thing.
What does matter is if you leave multiple blank. Even if there's a party you hate and you'd "never" vote for, in the situation where one of them is inevitably going to be elected, maybe you have a preference which one gets the seat? Fully ranking everyone, including that party as your bottom candidates, makes sure you always vote against them if there's anyone else, and you get a say in a last round determining which one you are going to stuck with.
STV is generally held to be possibly the best voting system, as long as voters understand how to cast their votes. That's the main problem - people are so used to weird voting systems going wrong and that their honest vote might be a vote in the "wrong way" to "let someone in" or whatever, that it's hard for people to accept that "no, really, just rank everyone in true order of preference", and that that really is "okay", and it won't have bad effects. (Quirks can arise, but nothing fundamental and predictable like the Reform/Conservative split effect that just happened, so not practically worth voters trying to be "tactical" about - a straight expression of real intent is best).
I'd say the main problem is that people may not be able to distinguish candidates, and their desired ranking may be like "1,2,3,meh,meh,meh,no,NO!". To fully express the "no,NO!", they do have to rank the 3 meh's. They can't do "1,2,3,4=,4=,4=,7,8", they will have to assign some arbitrary order to the "meh"s. When voters know they have to do this, the danger is they end up in ballot order, so the highest-place advantage can be amplified by lots of "meh" voters in a big STV list. Some places will randomise ballot papers to avoid that.