I suspect that, initially, the new channel will probably see a high amount of chatter of various forms. But, after a while, the channel will find its "niche" just like very other channel does. So, we will have to wait and see what "niche" the new channel actually fills before Turbine decides to give us additional channels. For some servers, one channel may be enough - for others additional channels may be needed. At this point, it is probably wise to test the waters with 1 channel.
If nothing else... this channel may more noticeably reveal to everyone who the "trolls" are on your server who are worth ignoring... and may lead to people wanting a bigger ignore list.
Instance Finder was tried... and, as is... is a dud. Look at all the complaints about people who use it but spend far too long in the queue to no avail. Look at all the newer folks who probably have no clue that GLFF or other global channels exist.
My point is... there is a chunk of the population that lacks an "on ramp" to entice them to even be interested... because there is no universal method of communication that they are likely to see.
Let's take a look at that oft-used quote from Sapience about "raiders/PvPers are less than 10% of the population". What if that number is so low because the other folks just aren't "plugged in" to the various methods raiders use to form raids? If you don't know where/how to find groups... how will you know groups are forming to join? I don't know about you, but I rarely (if ever) see the GLFF channel being advertized. I know of more people who actively avoid that channel than who ask if such a channel even exists.
Putting the quality/amount of group content aside for a moment... a good step toward improving/growing interest in grouping might be to give people a channel that connects them wherever they are on the server... and now that those people *know* about the instances LOTRO has... and are showing a genuine interest in doing more of them... I would think that gives Turbine more reason to invest in that kind of content.
At least... that would be my desired result of a global channel.