Satoshi cared that people using Javascript (?) or other languages which encode all numbers as 'double' (64-bit floats) would not have to jump through hoops to avoid stupid accounting mistakes. 

Your 64-bit float has 52 bits of mantissa, so, in order to avoid rounding errors ever going the wrong way, the number of units involved in bitcoin-related math must never be more than 2⁵¹, which is 2251799813685248 units.  21 million coins times 10⁸ divisions (Satoshis as they are now called) is 2100000000000000 units - comfortably just below the limit allowing "simple" accounting implementations in such languages to be accurate.

And as Hal Finney pointed out in 2008, even if the entire M1 money supply of the world as of that time were expressed in Bitcoins, the smallest division would still be worth less than 1 USAmerican penny, so there is no need for more than that many units.