The hoplites are indeed good units. They become obsolete only with the advent of rifles. (Anyone who would prefer to pay 60 shields for muskets when you can get hoplites at 20 shields is insane).
I think that the power of defense is underestimated by many. Having such cheap, great defensive units gives a substantial advantage.
However, I do prefer impi to hoplites. Their movement is more valuable than better defense, and if a town on your road network is threatened, you can move a swarm of impi to defend it in a flash. Impi can also be used offensively: stack them with horses for a 'virtual' 2/2/2 unit. If an attack against a city is close, the impi can themselves attack if necessary.
Also, the pillaging power of both the hoplite and the impi is awesome. The impi are great for trying to reach resources and snip the roads to them, as well as just leaving a general path of devestation.
-Sirp.