I have personally run into this situation where I had an Intel-based raid-enabled chip (an ICH-10R if memory serves) that was in AHCI mode, but I wanted to add a raid array.
To do so, I had to manually extract the Intel driver, override Windows' choice of driver, reboot and change to RAID in the bios. Done it a couple times, and always say a little prayer before exiting the bios.
The only real reason I can think of besides the longer boot time is that a lot of RAID configurations used to (still do?) require that you download a RAID driver for use during the Windows install.
Good point though, if you have any inkling that you might one day use RAID, may as well save yourself the hassle.