> It's not all that interesting because once you start going in this directions you can never stop. Quite often intervals are not enough
This is the opposite IMHO, most of the time the precision you have with floating point is good enough, that's why nobody care about intervals, but without intervals when (very rarely) you have a precision issue, you don't notice it which can be very problematic as in your example if you're trying to do 1/x on x=0.1+-0.2 with floating points you have the result 10 and you're (mistakenly) happy, with intervals you can see that there is an issue.
Of course, the discussions about interval arithmetic talk about the potential issues of interval arithmetic where your computation are precise enough but the interval is too big (because you wrote x*x instead of x^2 for example), this doesn't mean that this happen often..
> guile gives you rationals (which are suitable for most purposes)
Unless you want to use "rare" things such as square root, logarithms, exponentials, cos/sin, etc?
As for CPU speed, I'm not sure that for common operation (+,-,*,/) floating point ranges are much slower than rationals (on x86s which have powerful CPUs)..