.. then all you need is to set a flag in the ISR when you reset TI and 'poll' that flag in the main.

It "makes sense" to do the transmit in main when the code to fetch the next byte is complex i.e. not just the next byte from a buffer. Sometimes it can be beneficial not to 'unpack' the record to be transmitted to a buffer but to save the space such a buffer would occupy. In such a case the transmit code can easily be 'heavier' that what is good for an ISR. BTW in such a case, instead of 'polling' just check the flag for each passage in the workloop.

however:
NEVER poll TI (or RI) directly in the main if the serial interrupt is enabled.

Erik