there are about 5 houses that I have seen in the area close to me (well, between me and school.....see them on the school run).....I think it is just 'cannot-be-botheredness' TBH. they leave them and leave them, then think whats the point as it is nearly time to put them back again!
We've got a neighbour who adorns his house with a slightly smaller scale Blackpool effect. He does take them down in January, but he leaves all the little white clips on the house! It looks dreadful! I'd rather he left the lights up, I think!
Aristocat, I could be you neighbour - I have some white twinkly lights that still hang outside, I have a couple of dcs and (to my neighbours who I don't know very well) we probably appear to be relatively healthy.
However the lights are still up there (albeit unused during the non-christmas period) because they are an absolute pain to put up and take down (literally in some cases when the nail goes into your thumb), and if you try to take the nails out it causes problems and holes that get bigger and bigger and then you need different holes, all not good for the brickwork and render. It is white string of lights on cream background so they are not too visible during their off season! We did try taking them down the first year, having put them up because the previous house owners had done it and left the nails. Just caused lots of problems and mess so was easier, safer and better for the house to leave them where they were.
And neither dh or I have been very well over the last couple of years, not obviously to outsiders but enough that it is one less job that needs to be done and it is lovely to have them there when ds1 sees them and gets excited as christmas approaches (long may he continued to be pleased by simple things at christmas ).
And this summer we have wound some twinkly white lights through some of the trees in our back garden - not sure what you would make of them as they are very pretty in the summer evenings, definitely not christmasy but come christmas time they will look nice too. And best bit about those is that they are solar ones so no worries about outside electricity - yippee.
I had a friend whos Dad decided that he really liked having a Christmas Tree, so much so that he just left it up. It wasn't laziness, just someone who wants to enjoy the Christmas cheer all year round. Maybe your neighbours just really like perennials.
We still have our roof lights up, but to be honest they're clipped to the guttering and you'd never notice they are there. They also took dh 3 hours with a very very tall ladder to put up, so they are staying where they are.
I'm always tempted to put them on when we have a bbq!