Evasion isn’t useless, as long as you can’t use a shield because of bow or pistol/cannon, evasion is a good way to decrease the total overall damage you recieve from physical attacks.
Another thing to note is that evasion not only negates all damage but also the negative side-effects like knockdown on successful dodge.
You can also boost the worth of your evasion by reducing the enemies accuracy, e.g. by placing Aukuras near it or using Hunters Pointing.
You must also not forget that you already get to a decent evasion rate with equipment (10-30% chance not to take any damage is a lot better than 0%) and skills. With the upcoming patches, it will become even easier to evade attacks, because of the new 350 equipments that can feature DEX/evasion as bonus stats regardless of armor type.
It’s just my guess but I think that with a well-geared Archer and/or a well-assembled party, you will be able to hit over 50% dodge rate.
About current viability within ET:
With Swift Step 12 (gem+Divine Might) and some base DEX (i.e. your base value + some equip boni through e.g. Max Petamion) you will be able to reach about 210 evasion (without Monstrance or any other non-related evasion boost).
The highest accuracy of a monster in ET is 192, so you’ll have at least 6,5% dodge rate without any further DEX investment.
However, there are only a few monsters with such high amounts of accuracy(32 of a total of 89 level 330 monsters with high accuracy), and most 330 monsters in ET have 141-170 accuracy, leaving you with at least 11% dodge rate.
If we include buffs like Monstrance and Aukuras, we can achieve at least 19% dodge rate even against the monsters with the highest accuracy in ET. That is before investing into DEX.
At 200 DEX, you’d reach 40% against the monsters with the highest accuracy in ET[with this setting including Aukuras and Monstrance + Swift Step level 12], which is a lot of damage reduction on top of your armor and defensive buffs.
Last but not least, judging from the Vikrus Boots showcased in the 360 cap preview, we can be quite sure that there will be a lot more equipment to boost your evasion even higher, so you can reach your desired 50-60% dodge rate with the right setting even without investing heavily into DEX.