These are an eternal necessity in our ever changing world. With nothing being constant, to achieve a certain goal, you have to rely on your assumptions.
And with so much riding on the assumptions you make, it will be most important to make an intelligent assumption. Assumption which is backed by correct and critical information. Assumption which would ideally yield you to expected result.
Assumptions during uncertain circumstances are a necessity, but assumptions on needless topics are certainly unwarranted. And in most cases, assumptions on such needless situations are wrong.