Mist energy is valuable, because you can use it to craft, play, etc. Three Rings doesn't want players having unlimited mist energy for free. You get 100 mist energy per day for free (and 100 mist energy per day on each paid account).

Edit:

What is the point of all accounts sharing mist on 1 computer AND limiting the amount of free accounts you can have?

On second thought, I don't understand your question. It is not the case that all accounts on one computer share mist; only the free accounts on a computer share mist. Also, is there really a limit to how many free accounts you can have? If so, then what is the limit, and why have you approached this limit?

the point of having two failsafes is if the first failsafe fails, obviously. Can never be too safe. Dem loopholes have a tendency to generate infinite money.

Open 2 accounts at once. Even if they are free, their mist won't be depleted in real time (as long as both clients are open, try crafting something with one and leaving the other alone) So the sharing part isn't perfect, we need a hard limit.

Conversely, the 3 accounts limit on it's own would give us 300 mist.
Then we could restrict it to only one account per computer, but brothers would be screwed.

So, we need both systems. It's still exploitable tho, to a lesser extent.

Ellthan-Cosmolion, I still don't understand the point. You're saying that Three Rings should allow behavior A, but only allows behavior B:

A. A player creates 20 free accounts, and then pays money on each one (but the last), converting them into paid accounts.

B. A player creates an account and pays money on it. He repeats this 19 times. Then he makes one last free account.

The only difference between these is the order of steps. In either scenario, the player ends up with 19 paid accounts and one free account. The player gets 20 mist energy streams and Three Rings makes money. If a player were really interested in doing plan A, then wouldn't he just do plan B instead? I mean, is Three Rings' account limitation, arbitrary as it may seem, really such a big deal to such a player? I think not.

Finally, I've come up with some reasons for limiting free accounts. One early adopter player could make hundreds of free accounts, snatching up desirable user names, and then sell the rights to these names (as in domain squatting). An incompetent player could mistakenly think that he has to create a new account each time he plays. A player could be careless in remembering his username and password, because "he can always make another one". Three Rings doesn't want a whole bunch of abandoned accounts littering their servers (any more than necessary), and this rule may be designed to lessen that.

But what is the limit on paid accounts? Do you know, Hexzyle?