It all depends on how often you vape. Im a heavy vaper so an ego lasts me half a day, sometimes longer but a 3.7v mod that uses the 18650 battery lasts three days. they cost $5 and after around a year when they need replaced, its just another $5 to replace it, not $10-$15 every few months on stock batts. The Ego actually is a "mod", just without a removable battery.
For looks, cost and vapor production, for ME, mods are the cats meow.
The BIGGEST advantage for me as well, is having that full powered charge all day. Stock batts lose charge from the first puff, whereas a bigger capacity batt (more mAH) will hold that 4.2v longer, thus giving that "fresh off the charger vape" all day long and only drops to 3.7v when its about to die. Stock batts are vaping well below 4.2v a few puffs into it and continues to dramatically drop off until dead a few hours later.
If 3.7v works for you, great! But for me, I just need that "extra" oomph!

Dont get me wrong, I still enjoy 3.7v, but only from a battery with 900mah or higher....
Also, the "ego" is fairly new. A year ago, we only had stock batts with 180 maH and the SD mod which was also 3.7v, only with 1000maH. Longer batt life was what everyone wanted, so more and more mods hit the market from there on.