I understand that there are people with different needs than mine. I think the key here is in the words, "Absolutely need." If I'm out of the house I always want to have my medical cards for me and my teenage children. On several occasions I've been out, either at work or wherever, and I've had to pick one of my kids up from school or even from a friends house and take them to the doctor/hospital. My kids are very active and sometimes they get hurt or sick. Stitches, broken elbow, broken hand, broken ankle, etc. A few months ago I needed my VA card because I got a scratched cornea at work. Not having my medical/VA cards at those times would've been a huge inconvenience, downright irresponsible to be honest. I can't imagine paying for everything I ever buy with a credit card only. I almost forgot about my AAA card. I don't use it often but if I don't carry it with me I can't get the service when I need it. I might as well not pay for it if I don't carry the card with me. I can't predict when I'll need any of my cards that I carry other than the store discount cards and I rarely leave the house solely for the purpose of going to one of those stores. So leaving them, or any of the other cards at home unless I know I'm going to need them isn't really an option for me. And it also looks pretty unprofessional to have a customer ask for a business card and have to tell them, "Sorry, I didn't think anyone would ask for one so I left them at home."
My wallet isn't bloated with cards and cash. It's a normal single fold style with the clear plastic insert. I keep it as thin as I can, but for me, there's no possible way I could get by with only three tiny slots in a phone case without being severely inconvenienced on a fairly regular basis. Come to think about it, I don't think I know any adults that could. Different strokes...