I used to pay for two World of Warcraft accounts, but the second one was for my wife, and that one is actually still active, because she is still playing. Having two accounts was useful for transferring goods from Horde to Alliance via the neutral auction house (can't do that with just one account, you can't buy from yourself). And one day I used the two accounts to unlock the Dire Maul door with one of my characters who had the key, put a character from the second account inside, and then used him to open the door with the lever for another character from the first account, who didn't have the key. But apart from that I barely ever touched the second account. There isn't much of a point in having two WoW accounts for yourself, unless you try to powerlevel yourself with dual-boxing.
I also had two accounts for Star Wars Galaxies. In that case the second account was for myself. I tried to get my wife to play it, she tried it, but didn't bite. But in SWG having more than one account for yourself was useful. For starters you weren't allowed to have alts, only one character per account. And then you could only place a limited number of structures like harvesters and factories with each character. So if you paid for two accounts, you doubled your economic output. EVE Online has a system in which skills are learned in real time, but only one character per account can do so. Again having more than one account could be an advantage.
One game of the future where I could imagine having more than one account would be Pirates of the Burning Sea, after reading that you are limited to 10 production buildings per account. That won't help with leveling up my character, but I would have twice the economic power, and could integrate larger chains of production, maybe even have most of the parts together to build ships.
Now some people already consider one monthly fee to be too much, and would never pay two of them. A second account for the same game always has diminishing returns, you don't get twice the fun out of two accounts. But even $30 per month is still less than some people pay for cigarettes. Not that I would want to pay for 40 accounts. But two accounts is something I can afford and what, depending on the game, I might still consider useful. What do you think?