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Orb of Storms is very good for both of those applications. It is perfectly fine as a 4L, links 5 and 6 give you more quality of life (from Inc Crit and Inc AoE).
They are by no means necessary. I have Ele Overload up almost every time I drop Orb of Storms. A 4L Orb with Ele Weakness and Flammability would definitely be viable.
If not 4L where else are you getting that crit chance for elemental overload?
Orb of Storms has a base 5% crit, so it won't trigger it every time without Inc Crit, but you can definitely keep it up most of the time. Grab a Diamond Flask for bosses and it will be fine.
