It becomes a habit when you write according to standard rules.

"The small numbers, such as whole numbers smaller than ten, should be spelled out."

http://www.dailywritingtips.com/10-rules-for-writing-numbers...

Edit: Also, I prefer to use proper English (to the best of my abilities) even when addressing my computers.

Mostly because it's natural to type a word longhand in the form of a sentence. So it's not that you should be able to do one or the other, but that you should be able to do either. A user shouldn't be coerced into doing something unnatural for them if your premise is to make it conform to a user's natural language rather than forcing a query type language.