I prompted myself to type this post up after a brief conversation I had with some Twitter friends, including fellow DA user vaslittlecrow, about online artists who specialize in fan art. The position we agreed on was that while there's nothing wrong with drawing fan art by itself (since so many of us have done it ourselves from time to time), specializing in it is overrated as a strategy for amateur artists to reach a large audience. Unfortunately Twitter isn't the most conducive medium to thoughtful in-depth discussions, so I'm expanding on this thesis here on my journal.
Truthfully I'm not even sold on the conventional wisdom that fan art is guaranteed attracts more attention from the online community is invariably true. It appears to hinge on the premise that people connect better to widely recognizable characters from popular culture than what individual artists create from their own imaginations. That has a kernel of accuracy insofar as some characters and stories receive more public exposure than others, but the problem is this doesn't always mean people genuinely like those intellectual properties more than their less broadcasted brethren.
The sad truth of modern pop culture is that it's far from ideally meritocratic. If anything, inducing public irritation or outrage reaps as much profit and cultural exposure as producing quality work; you're just as likely to see infamously bad books and movies sell for mountains of money as you are books and movies that everyone likes. Notoriety has practically become the currency of the 21st century. What this means for the amateur art community is that the characters who attract the most attention won't always be the solid, multidimensional ones that everyone would earnestly like, but simply the "trendy" ones supported by today's most aggressive advertising campaign. Lots of quality characters from quality stories get shoved away from the spotlight, so even the most heartfelt fan art celebrating them is at a disadvantage compared to art featuring whatever gets promoted the most at the time.
In conclusion, drawing fan art as a specialty probably has less power to attract a broad online audience than many in the amateur art community claim. The strategy would only work if fan artists just drew what was currently the most visible (but not necessarily the most well-received) than what they truly enjoyed themselves, but what amateur artist really wants to do that?