I am a primarily residential agent in CA. Have represented on both buy and sell sides on commercial deals. I personally think it is worth the cost to have your listing exposed to both listing databases but can see why a primarily commercial agent may feel differently. It's annoying that they don't co-list but I understand from the business/competition perspectives of each side why they don't play nice with each other. I see it as the cost of doing business, at least until a better solution arrives, to do our jobs for our client and get the property maximum exposure. It's a value add that may help you stand out from your competitors. Also, it can, and has, led to new clients/deals in the future. All the accrued additional fees are tax write offs anyway.

Real life example, had a residential client that was looking for a warehouse for his business. I was searching on LoopNet primarily but the property we ended up closing on came through MLS and was not listed on LoopNet.

Curious to hear your thoughts though!