Prepositions are generally very small words ("of," "to," "at, "from," etc.), but they are important ones:

The candidate compared his opponent with a shady used car salesman.

Problem and cure: It is standard to use the phrase "compared . . . to" in this case.  Why?  Because our idiom has established the norm.  Standards of idiom do change over time, but this is not to say that anything goes when it comes to usage.

Note how the substitution of prepositions radically alters the meanings of the following sentences:

Dennis threw the ball to me.
Dennis threw the ball at me.

I have worked with her before.
I have worked for her before.

common errors