I do agree with the point Valkyrie makes, its not that I find ED boring but rather, why would I do X or Y activity only to get to a ship I like less, but does the same activity more efficiently?
I am willing to live with the lower efficiency of the small ship, really. If it were only about kill rate, I'd stay in the Eagle until I got a Courier, and skip the Viper entirely.
The real reason to upgrade to the bigger fighters is survivability. The Viper is MUCH tougher than the Eagle (partly due to shields and partly due to the extra utility mount), and the Courier and Vulture are much tougher than the Viper. This becomes important when huntimg big game like Elite Anacondas.