I think the main idea behind the PgMP is to promote PMI's standards in program management (regardless of whether these standards are right or wrong). That's from PMI's perspective. As for program managers seeking and earning the PgMP, it's all about prestige, as there is not many PgMPs at the moment so it looks like an exclusive club.
Now let's examine whether the PgMP has value or not from several perspectives:
Work: Unlike project managers, program managers do not need to get certified to be hired. A program manager has an executive position in a company, and does not actively seek jobs, in fact, in most cases, he is approached by competitors to work for him. There are not a lot of program management jobs out there, but there is a very scarce supply of program managers, so there's always more demand than there is supply (A person needs a lot of experience in order to become a program manager). Quick question: When was the last time you saw a job ad for a program manager requiring him to have a PgMP?
Salary: Again, program managers have always a reason to negotiate their salaries, and that reason is never that they have a new and shiny certification. They know and their employers know that this certification won't do anything new for their organization.
Experience and skills: One cannot become a program manager without having a lot of project/program management experience. That experience will not be enhanced by a certification issued by a project management body, claiming to standardize program management. Every program manager is mature enough and experienced enough to manage programs in his own style, and he doesn't need PMI to tell him how.
Standardization: I think this might be the only real value for the PgMP, as it implements standards in program management, which means that if there is a PgMP certified program manager handing over a program to another PgMP, they might be on the same wavelength, and the handover process won't be that hard. But this is, of course, a panacea, and again, who says that PMI's standards when it comes to program management are written in stone.
The only value (besides standardization, if you think of it as a value) that I see for the PgMP is the prestige, and that's it, simply because there are very few program managers that currently hold it. Now the question is, is it because this certification is very hard? Or is it because they don't see any value in it? I personally go for the latter.