If we're talking about aerodynamics for triathletes, the main advantage of a tailless aero helmet over a tailed-style is that you don't incur an aero penalty when you change your head position while riding. In particular, if you lower your head (to look at a computer, to grab a water bottle, to rest your neck), the tailless helmet won't increase your drag. If you're using a tailed helmet, the gap between the tail and your back will open, causing extra turbulence, extra drag. When we're talking about some of the distances that triathletes routinely race, most people I've spoken to, including myself (yeah, I talk to myself. It's not creepy) agree that we don't keep perfect head position all the time on the bike.
Speaking more subjectively, I believe many people prefer the look of the tailless helmets, especially the aero road helmets, because they look less like "serious" race equipment. They feel less self-conscious doing a training ride with a tailless helmet like the Giro Air Attack than they would with a tailed helmet like the Giro Selector.
Below are two examples of aero road helmets. Giro makes the Air Attack Shield, which has a removable/flippable visor (this also increases the aero effect). Catlike makes a closed-vent version of their Mixino road helmet, called the Mixino VD 2.0. Both mimic the size and shape of regular road helmets, but have fewer vents.
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| Giro Air Attack Shield |
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| Catlike Mixino VD 2.0 |
And here we have two examples of the snub-tailed aero helmet. Notice the vestigial tail, more suited, as Rudy Project says, to multiple head positions.
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| Kask Bambino |
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| Rudy Project Wing57 |
