To program it with the uart, you are likely to need to pre - program it properly with a boot loader anyway.
Once a bootloader is installed, the advantages are quite clear, first off you can use usb to serial adapters to make it easier to use on new computers. You can also use bluetooth to serial. You can use easy radio modules and most other radio modules to program with. So realistically you can program your robot wirelessly if you have the correct hardwares meaning that it is a lot easier. When I have a new program to load onto any of my robots, I usually have to load it on, disconnect it to test then reconnect it download modified code, disconnect it, test it, reconnect it, download modified code, diconnect it, test it, etc... etc.. It is a lot easier if you dont have to physically connect you robot to download new code. ps. i learned my lesson and invested in some radio modules
The downside is losing a bit of programming space for the bootloader to reside in