Perhaps the RTS toggle after transmit delay needs to be large, and 
they'd rather do it in userspace than block in the driver. I also
recall a protocol that would send a master assertion command and hold
on to RTS afterwards. I can easily imagine needing to quickly

Appletalk does this for one. Some protocols use it for priming low cpu
power receivers. You fire a header message at the target which fits
into its serial fifo, it then responds and can sit in a tight loop
getting the following frame data.

Alan