IMHO it's not a bad idea to have a USK but you need to keep it small and unobtrusive (15 lb pack on your back is a bit much). If you really feel you need a lot of stuff very near your person, try keeping multiple kits. Something small for your person, a little bigger (like the backpack) for your car, and even larger kits and stores in your home or other buildings you frequent and are allowed to store things in.
Keeping the kit on your person small is important for multiple reasons. You probably don't really want to be carrying a big backpack mostly because it's impractical. Going about your daily business it's just one more thing you have to keep track of and it will attract a large amount of attention compared to a smaller bag or no bag at all. Something like this might also make you a target if you were to be running around with it. Other survivors, in a survival situation, and criminals, in a normal situation, may see a giant backpack as a great potential target. You want to make yourself stand out as little as possible in these situations.
Just some food for thought!
Keeping the kit on your person small is important for multiple reasons. You probably don't really want to be carrying a big backpack mostly because it's impractical. Going about your daily business it's just one more thing you have to keep track of and it will attract a large amount of attention compared to a smaller bag or no bag at all. Something like this might also make you a target if you were to be running around with it. Other survivors, in a survival situation, and criminals, in a normal situation, may see a giant backpack as a great potential target. You want to make yourself stand out as little as possible in these situations.
Just some food for thought!