Personally? I prefer to pay for the things I want to pay for. Some flights I want to have addtl leg room, or check a bag. There are times I don't care where I sit, so not paying to book my seat when I book wouldn't much matter...at least, not all the time.
I much prefer to be able to make a less expensive flight if I choose not to get addtl leg room, check a bag, or sit in an aisle seat.
My issue?? The airlines that make every single desirable thing addlt. For instance...when my dh (6'4'') and I flew to San Diego last fall, I had to pay addlt for him to sit in an aisle seat! I got to sit in the middle for no addtl charge. But, if my dd had flown with us?? That window seat would have been addtl as well. So, two people fly together, but one of them has to pay extra. I guess you could also look at it as the person choosing the middle seat is getting a break. All in the way you look at it.
But if my dh had really wanted extra legroom and an aisle seat??? Cost was much, much higher.