[IanCal]

You might be over-estimating the complexity of the kinds of problems generally used for this.

You're not bothering people while they're taking an expected amount of time to look at it. You're stopping a several minute silence after handing someone a basic loop and if statement.

For most positions we're talking about, this should not be a major problem. I found when doing something similar people either looked at me like I was crazy for giving them such a basic problem and after being reassured there was no catch answered simply and clearly, or just couldn't do anything. Followup questions about the code then went terribly (e.g. a JS contractor asking for £500+/day who didn't know what the difference between global and local variables was, why you should put var in front of things, etc).

These things are often good to weed out the incredible number of people who seem to have little to no coding ability at all for jobs where that's an obvious pre-requisite. I've also had people fail to solve a basic problem (roman numeral generator or reader) at home in whatever language they wanted, not even getting vaguely close. One person even sent in their broken version pasted into a word document.