I've also been using that option for all the new releases, and it hasn't caused any problems for me. The question is, if this option only increases compatibility without any drawbacks, why can you turn it off? Shouldn't it at least be enabled by default?
Maybe it changes a few things like soft reset that might affect something like download play. just a hunch. I only use it when it won't work without it.
quite simply, no there isn't a downside, it's just to force read/write from the flashcart instead of slot1 on some games that's adamant about working from slot1 without the new software being out already to properly patch the game.
I've had to use the force r/w option to get several recent games (including NFS Carbon) to work on my G6. I hate having to reload the games with different options, so I usually turn force r/w on just in case it's needed.
The M3 doesn't use the same software as the G6 does it? Perhaps the M3 loader/flasher is better in this regard.
Rumiko Takahashi got so much fan email about the character that she brought her back in the manga, it was going to be completely different originally (with the alien of the week type).