Just my opinion, but no one, including a Trek tech, can guarantee that a bicycle will never have a chain slip...get it in writing/speak into my microphone dealio on that conversation! I have had the fix he mentioned and afterwards my chain slipped.
My take is the only reason for having the chainguard is to prevent clothing from getting caught in the chain. I have never had this happen to me but chain slips, yes. The chainguard does not protect the mid drive motor since it is factory sealed and it certainly does not provide much of a chain guide, hence all the chain slips. I can see no functional reason for having it. Also, between you and me, I think it looks rather crappy/chintzy. I actually like the look of my bike with it off, it looks more like a real bike and less like a Cadillac Escalade.
As mentioned previously the Verve models are the only one in the Trek e bike line to have this. I think the Trek designers were perhaps looking at the Verve as a commuter bike and the chainguard is to protect someone's silk pants on their way to work. For me, this was not a concern so off it came.
Just my 2 cents
Howard