Surtr is designed to be a "helidrop" like SA and Eyja, except she's forced to retreat with her S3. If you want to keep her on field by using S2 over S3 that's also a strat but not recommended.

Main reasons people don't use S2 is because 1) less damage, less range and less attack count meaning overall lower dps and 2) her S2 is basically another form of Blaze S2 just with arts and not infinite duration.

Her S1 is fun if you can find the correct enemies. Eg her original attack is 550 with S1M3 that's 1650. She can usually one shot 500 HP enemies but with S1 she can now constantly one shot 1500 HP enemies. You can also think of this skill as BP S1.