The reasons to pay for subs because that money helps pay for content. I have never played a MMO that did not require a sub. Where people get this idea that you need to only to pay to install the game and that's it I don't know. MMO's over changing content because the game constantly changes, content, class balance, zones, ect...
I understand the F2P model and no, just because you choose to be F2P does not mean you should have a free ride for all content because you are not supporting the game unless you actually spend money.
10.00 a month is not that much considering how many hours of content you get out of the game. You should have to be premium to get 100% of what the game offers. I can buy a soda and some candy at 7-11 and spend the same 10.00 for something that lasts me 10 minutes.
It's all about value. For not buying anything, Funcom is giving you a taste of great content that you can spend weeks enjoying. If you want more, you'll have to spend some but you can unlock content as needed.
Or you can just spend your 10.00 a month, buy a yearly sub, and enjoy countless hours for 365 days at the price of a soda and a box of donuts every month. Easy pick for me.