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

As long as you get a rare pokemon that you or many other people do not have , sure it is.

As dly said it's about candies, filling pokedex and high IVs, but on higher levels the thing that really matters is IVs, because you probably will have tons of candies and filled pokedex(well, may be without legendaries). And you will look for pokemon with 90%+ or 95%+ IVs(attack, defense and HP are almost 15) which are much more likely obtained from eggs. 90%+ IVs pokemon are a really rare thing to catch.