You wrote : "Generally, if you cannot provide a motive, juries have a difficult time convicting."
What you wrote is generally NOT TRUE nor should it be if you have an intelligent, well functioning jury. If your evidence is not strong enough to convict without having a motive then you do not deserve to get a conviction. There are many times when guilty parties cannot even tell you themselves their motive or "why" they did a crime.
You're right. Motive is circumstantial justification for suspicion. It's more useful in building a case for an arrest warrant than in building a case for conviction where other evidence exists. Motive is very helpful to a prosecutor trying to get a conviction on circumstantial evidence alone, but it isn't at all crucial in a case which includes physical evidence and sworn testimony.