The reason someone serious about embedded development would want to buy Arduino boards is that they're convenient Atmel-architecture microcontroller boards.
The ESP8266 may be "arduino compatible", but a large number of those Arduino libraries running on it had to be rewritten.
If you want convenience and ISA/peripheral-register compatibility with existing low-level Arduino code, you're locked in to atmegas unless you want to do some fun porting work ;)
If you want convenience and ISA/peripheral-register compatibility with existing low-level Arduino code, you're locked in to atmegas unless you want to do some fun porting work ;)