As for the pile-of-poop emoji there is a very good reason for it to be included in Unicode: it is one of the original emoji, made by Softbank in 1997.
The reason: Softbank and Docomo both used proprietary emoji sets, widely used in Japan for text messages. Not including these in Unicode would have resulted in a choice: be compatible with the rest of the world, or keep using the emojis that became part of their culture. Not satisfactory. So basically, they had to put emoji in Unicode for the Japanese to use it, and excluding Japan is not really an option if you want a universal standard.
SoftBank actually put emoji into the Private Use Area of Unicode before they were codified (just as they were in an unused area of Shift-JIS before that). The original iPhone 3GS use the PUA method.
The encoding of emoji was in order to unify Softbank/docomo/au under one set so that iPhones/Androids sold by the different carriers could send emoji among each other without relying on email translators (as had been the technical solution with feature phones until then)
It made total sense to include emojis in Unicode initially.
And a great idea to put them outside the 16-bit plane, so platforms would be forced to support characters beyond the BMP because of the public demand for emojis.
But I think going forward, emojis should be handled outside of Unicode. They are really small illustrations rather than characters or symbols.
The reason: Softbank and Docomo both used proprietary emoji sets, widely used in Japan for text messages. Not including these in Unicode would have resulted in a choice: be compatible with the rest of the world, or keep using the emojis that became part of their culture. Not satisfactory. So basically, they had to put emoji in Unicode for the Japanese to use it, and excluding Japan is not really an option if you want a universal standard.
It did open an Pandora box though.