In the bitcoin context, the 'SI purists" insist on the decimal point staying where it is, forcing everyone to deal with currency to 8 decimal places, making daily mental arithmetic impractical.
The 'bits' advocates want the decimal place moved 6 places right to a more standardised currency convention and format, it works better with financial software, and more importantly it better suits how our brains have evolved in dealing with 'counting numbers', it helps bridge bitcoins usability chasm. Having moved the decimal place, SI can operate as per normal. It's still a decimal system. It still has a decimal place.
Not only that but behind the user-interface level, the 'SI purists' can talk to each other in micro or milli all day long if they wish, so they have nothing to lose, and everything to gain.
You do realize that if I talk about 100 mBTC I've effectively MOVED the decimal place, right? Do you really think I would pronounce this Oh point oh oh oh one bitcoins when it clearly can be read One hundred mikes? (using mike as an abbreviation because obviously the word microbitcoins is too long)