The only thing that keeps me from carrying a multi-tool is I can't leave it on my bike as it will get stolen. I carry multiple bags depending on the weather, whether I'm commuting or running errands so the likelihood of me not having my multi-tool in my bag is significant. The only option I have that I'll dependably not forget are inadequate keyring tools. Make a tool that mounts to the outside of a u-lock hoop and can't be removed without opening the u-lock.
The Brompton multi-tool that fits in the folding tube of the Brompton is the best scenario assuming you take the bike inside with you instead of locking it up.