I was going to put up a comment on Adric's blog responding to the statement, but it got too TL;DR-ish, so I'll put my thoughts down below(don't worry, I'll try to keep it concise and brief).
I think the answer isn't in the middle, but rather that the answer is all three due to the fact that there are viewers that fit in each category. A spectrum, if you will. For example, let's look at viewers like Hippo, Imprudence, Meerkat and Snowglobe. The first three have third-party grid login management far superior to the official viewer's clunky "--loginuri=
Then there's the undeniably malicious viewers. First it was Copybot, now we have an unholy trinity of Cryolife, Thuglyfe and Neillife. All three are at their core a continuation of CopyBot's malicious intent(copy objects with no regard for the Second Life permissions system). Thuglyfe goes a step further and includes features designed to avoid parcel and estate bans. These are the kinds of viewers, open source or not, that should be fought against vigilantly.
And currently straddling the middle of the spectrum is Emerald, the most popular but controversial third-party viewer(Disclaimer: I have used Emerald, but as of late I have been using Snowglobe and Imprudence instead). There are so many cool features of Emerald I can't list them without turning this paragraph into TL;DR, so I'll go straight to some of the controversial features. First there's de-friending notification. It's okay in SL to de-friend someone and letting it pass quietly. Getting a notification that you've been de-friended is a quick way to cause drama and grief. Showing real online status and avatar keys in profiles can also be griefing-enablers.
Also the past activities of some Emerald developers don't help their viewer. However, the Emerald developers seem to be willing to work with Linden Lab on viewer policy, and they are now keen on combating malicious Emerald derivatives such as Neillife, so the jury is still out on Emerald. It can go either way.
So to either wholly allow all viewers or condemn them as Prok does are completely irresponsible and one-sided views. It will lead to either rampant copyright infringement and crime or feature stagnation, greatly harming the value of SL. A balanced view of the spectrum, however, will ensure that Second Life and it's derivatives will continue to succeed for a very long time.