I think one or two short ones are fine so guests have a breather and can get a drink if they don't feel like dancing a slow song or don't have anyone to dance it with. However, this can clear the dance floor depending on the crowd and having a good DJ be able to read the crowd and fade out the music to a more upbeat song if needed is very helpful.
Yes! Weddings are a romantic event! Your guests might get the feels and want to slow dance with their date. It definitely gives people a breather, too.
Some slow songs are nice to break up the upbeat dancing, give people a chance to catch their breath / rest, and couples a chance to dance together lovingly.
We didn't have any slow songs besides our first dance. This gave my MIL and FIL an excuse to not dance at all! Definitely know your crowd. In hindsight, I would request at least a couple for our older relatives in attendance.
Yes you need slow songs. Plus other guests like to dance to slow songs as well. Depending on the length of your event more than 2 is perfectly fine. We aren't just having 2.
I would put a slow song, 4-6 upbeat, slow, 4-6 upbeat. Lots of people (generally a little older) would prefer to slow dance so I think it's nice to sprinkle them in at least for the first half of the dance portion!