It is that time of year when it is often very cool in the morning and quite warm later in the day. For many the answer to this problem is layering. I find that that isn't any kind of answer.
I have no aesthetic objection to sweaters over shirts or anything like that. The problem, for me, is that a shirt that looks good with a sweater or a vest over it may not look good when you get rid of that top element. I don't like the look of high necklines (on me) for example but that's the kind of top I wear under things.
Furthermore, if it's the right temperature for a particular sweater, wearing that sweater with something under it makes it too warm. Several days last week it was cold enough in the morning (and in the house because, let's face it, no one puts the heat on in September if they can help it) to wear a chunky sweater. Later in the day it was too hot, especially outside in the sun. Because I tend to only get dressed once, I just suffered. (In the winter, I would have worn that sweater all day and then put a coat over it to go outside.)
Often for this kind of cool but not cold weather the best option is to add some kind of thing under-layer. Getting rid of that layer later in the day is also kind of problematic, especially if you are not at home. It's not like taking off a cardigan.
This only really becomes problematic when I'm buying/making new clothes. Neckline choices depend on how I'm going to wear something.
I recently bought some long sleeved t-shirts. If I was just going to wear them alone, I'd go for a scoop or v-neck. But that limits the season in which I can wear them rather a lot. Realistically, most of the winter, I'm going to put a vest or a cardigan over that t-shirt and keep it on all day. That means the scoop or v-neck is in the vest/cardigan and I need a neckline on the t-shirt that complements that. Turtlenecks and crew necks are what I bought.
Of course then I felt like I still didn't have anything to wear in those very short periods of the year when I want one light long-sleeved layer.