Are Pokemon from eggs still useful at lvl 30?

Basically you won't get better Pokémon after reaching level 20 due to this:

  TemplateId: "PLAYER_LEVEL_SETTINGS"
  PlayerLevel {
    RankNum: (...)
    RequiredExp: (...)
    CpMultiplier: (...)
    MaxEggPlayerLevel: 20
    MaxEncounterPlayerLevel: 30
  }

But(!):

• Rare Pokémon
• Pokémon with neat values (this for example)
• Candy
• More Candy
• Filling your Pokédex
• Even more Candy

So yes, hatching eggs is still useful until you completed your Pokédex and powered up everything. Especially collecting (and using) Candy will be a pain in higher levels, since the power up costs will be pretty high.

First of all, define useful.

It depends on you, if you want to catch'em all or if you want the strongest one. Or maybe you want to evolve Pokemon.

• Evolve: eggs are useful. It gives you a good amount of candy. See: How many candies do you get for every 2k, 5k, and 10k egg?

• Strongest: it depends on when did you get the egg, not when hatched.

• Catch'em all: You can get 73 pokemon by egg