Yes you need to assume perma burning in order to make fire traits desirable for damage itself (obviously the trait line is desirable for persisting flames). But if you take the stats and the modifiers (300 power, two 0.05 mods on burning ) compared to only the stats of the air trait line then you get a slightly lower increase to damage under full buffs.
Zelyhn.8069
[rT]
Retired elementalist theorycrafter