Making it a full-members only feature may encourage others to upgrade though, and so would be worth the while.
That's your key point methinks... More and more sites these days offer some degree of mobile connection so if you don't move with the times, you'll be left behind.
Mobile technology's getting better all the time and the majority of phone contracts offer some sort of interweb bundle in your monthly services.
It's another gamble, months of development against the potential full membership take-up, but it's a very quantifiable benefit to paid membership, so it may encourage folk to part with the pennies