Arfur
I'm with Subox - if you're willing to pay, why not sub?
Good question. If you sub, your account is locked into P2P mode and you can't go back to F2P. Some people like freedom and flexibility and don't want to get locked in.
Imagine a college student who wants to play a lot during the summer and over winter break, but only intermittently during the regular semesters. Perhaps this player would like to be able to log in every now and then to touch base with his friends, but can't financially justify subbing for a full year if he's only playing seriously for 3-4 months out of the year. If this guy subs for only 3-4 months, he doesn't have access to his character for the other 8-9 months of the year. The bottom line is that F2P gives the player the flexibility he wants, and P2P does not. However, this player might otherwise be willing to pay for benefits that won't lock him out of his account.
Here's a relevant idea that I posted in another thread:
Yuengblood
Make a "subscription-lite" plan that costs something like 75% of the regular subscription price, but still has the level cap and storage limitations of F2P.
While paying this fee, the player gets the 2X xp bonus. When "subscription-lite" expires, the account reverts back to regular F2P with the 1X xp.
This would allow WG to collect money from "F2P'ers" that would otherwise be reluctant to lock their toons into a P2P account.