I find the biggest reason I went premium was to support the site. The wealth of information you get from geocaching.com is insurmountable compared to the small $3 monthy, or the $2.50 per month for annual membership.
It's not the .COM era any more folks. If we don't support these guys, they'll stop making money, have no investors, and eventually go under like 90% of the .COM era sites did.
If you're an avid, or even moderately active cacher, and you don't have a premium membership, you should ask yourself why not. Some magazine subscriptions cost 2x, 3x, or even 4x what an annual membership runs here.
Thanks for reading.