The WT's reference to partakers as 'proud, mistaken or mentally unstable' is a method of 'framing the argument.' See my post here (on another site) for a link to that type of strategy.
The idea is to establish the parameters (or sides) in the debate: There is WT's view (which is presumed to be correct), OR, you are proud, mental or have some other bad/undesirable characteristic. It is one side or the other. There is no other option (according to the WT's framing).
What they NEVER mention is the fact that a growing number of JWs are partaking because they believe that this is what Jesus wanted them to do. And a growing number of them are doing it in private meetings apart from the WT. The WT does not want the R&F to consider this possibility, so it is never mentioned.
I would think that they have woven the idea of 'partaking' into their power/authority structure. So they see the idea of others partaking (and its implications for the partakers) as a threat to their authority. In the gospel accounts, Jesus is presented as a similar threat to the then existing Jewish religious authority.