With TW in server and gateway mode, Mitel has concerns of security exposure with these 2 applications being co-resident.
My opinion, the security in the MAS is quite reasonable and Mitel is just covering their butts.
With TW in Server only mode, I'm not so sure as I don't know much about the security in this instance and the system is already behind the firewall. At a guess, this would be a greater risk of security breach if the App's were co-resident.
Other than security, these Apps work quite well together.
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Occam's Razor - All things being equal, the simplest solution is the right one.