Pros and Cons of Making the game Pay-to-Play

What's the point of paying monthly fee if you still have to pay for stuff?

Exactly. If Nexon went P2P (which they will never, ever do), the would have to at a minimum:
1. Hire more customer service staff
There is a huge difference in the level of GM access and customer service response between what Maple has and P2P games. When I was playing SWTOR and got stuck in a map glitch, a live GM responded to my ticket and teleported me out of the spot, and logged the incident for developer fix in under and hour. I was shocked and amazed.
2. Make Regular store permits free
There's a level of expectation for "quality of life" in a P2P MMO. No one get's away with this kind of pomegranate when you already have to pay a monthly fee.
3. Monthly allotment of cubes (debatable amount/type)
If you're paying a monthly fee, you should get something for your money even if it's determined by RNG.
4. Faster developer response time
Nexon is no where near industry standard for bug/glitch/exploit response time for P2P games.

I could probably think of a lot more things that might need to be changed, but it get's rather subjective. My point is that MapleStory does not and cannot compete with P2P MMOs in it's current state. Not to mention having Nexon transition it's flagship game to P2P would cause a huge investor panic because it would indicate that the F2P model they "pioneered" has failed.

EDIT: If by definition the subscription "saves" you money, Nexon is losing money.