AFAIK, in N8x0 days, or at least this is what I know about others, R&D mode allowed you to wipe the bootloader causing the device to be completely unrepairable. Not sure if this is the same with the N900 since it includes USB cold flashing, but it might be similar in some ways. On that note, been in R&D mode since May iirc, your battery life might decrease a little (10 minutes maybe?) because of the lights (but those can be turned off)