I am sorry, with all due respect but this is how I look at it.
Is it unreasonable? yes.
Is it possible for the event to occur? yes.
How bad it is, if it happens? very bad.
I need a solution for this.
Specially consider the cases where the user is going to enter a number. Or even some part of the code, some how manage to produce a very large number, due to mistakes in code (debugging) or hardware error/faults (disaster).
Let's say a sensor connection come loose and the ADC reading drops to a level it would never happen if the sensor was attached to it. And for some reasons a constant is divided by this float variable coming out of sensor. Then your dtostrf is going to mess the whole memory. Put it in a expensive project with mechanical stuff involved and then things may not look that good.