The main disadvantage of this that I see is such small geo-squares being hard to remember. 10x10 meters is a small size. To remember those by hard without writing any coordinates/pointers down or saving related information in a digital form would be too hard. It's easy to remember a huge location like a city or a whole neighborhood or some famous object like a cathedral. A combination of those might work, but then the system would become too simple security-wise. To remember some 10x10 square something memorable has to be inside it.
What can fit into a 10x10 square - a tree maybe, or some sculpture, but if another tree is standing nearby you might confuse those easily after not visiting that spot for a while. Landscapes tend to change, especially in bigger cities. Imagine some café, bar or shop was in your square, that's easy to remember, but in a big city that establishment might close and another one might get opened, the building might get restored or even demolished, etc.
Also, in big cities, there would be many flats over the same square because of tall, multi-floored buildings. This might become an issue for such a system if a few people often chose the same square because of some familiar flat or office being over it. Even if your square is in the middle of the field with some lonesome tree inside it the tree might not be there after some time, how will you recognize it then? The only way to secure this will be knowing the coordinates of the square, so memorizing words will turn into memorizing numbers.
Also, even if the same place happened to not change for a long time (some particular public bench or whatever), the places that you consider important, memorable could be guessed from your online activity rather easily with the help of a specialized algorithm – if you happen to like the place you’d visit it/pass by more often, mention something related in chats, etc. Alternatively, you could try to discover “pass-places” on google earth, but this is again not a hacking-proof option.