i have 2 bags for bunjee, one purse-like bag and one messenger-style bag. although i like to walk bunjee as much as possible, it's sometimes easier having him in a carrier and a stroller is just too big in my opinion.
i take the bus a lot and although dogs are allowed on buses, they have to be muzzled, even a 7lb little one like mine. and when the bus is packed, i run the risk of having someone step on my poor little bunjee. rather than putting bunjee in that kind of risk or having him muzzled, i'd rather have him in the carrier.
i also go downtown sometimes where foot traffic is busy. there's parks downtown where i can walk him and stores i can walk him into, but to get to them, there's a lot of foot traffic i need to pass through... again rather than risking someone not seeing him and stepping on him, a carrier is much easier.
having him in a carrier even at a pet friendly store also reduces the possibility of a potty accident. i've even taken him with me to a spa once while i was getting waxed. my wax lady was really nice and understood why i didn't want to leave him in the car for an hour and was totally fine with him being there so long as he stays in his carrier.
my carrier also doubles as a car seat for him when i'm driving and on the go. and when i'm on an airplane, a carrier is a must in order to take him in cabin with me. now bunjee doesn't necessarily like being in the carrier and will struggle a bit until i zip the carrier shut. but he's perfectly quiet and well behaved when he's in his carrier. no one even knows i have him in the carrier until i take him out of the carrier and start to walk him.
i guessimated his adult size when he was 12 weeks old and bought a larger size if he was in between sizes. i think it's more comfortable for him with more space. i just stuffed the carrier with extra towels and blankets to make it smaller until he was ready for more space.