The main problem is that floating point is not sufficient to accurately store a Bitcoin balance
An IEEE double-precision floating point number has 53 bits of precision, which IS sufficiently accurate to store a bitcoin balance.
Every single possible bitcoin value can be converted to and from an IEEE 64-bit float with no loss of precision.
I agree that if you're going to be performing lots of calculations on bitcoin values you need a Decimal type (and ClearCoin stores and uses python's decimal.Decimal(precision=8) for all bitcoin values)-- if you don't, floating point errors can accumulate and eventually cause you to gain or lose .00000001 of a coin.
But really the main problem with storing monetary values as any floating point type is you're likely to be embarrassed by mistakes like error's cash register receipt if you truncate values instead of rounding before printing.