I understand your point, and I agree to a certain extent. At first I thought the same way. I respected the diversity and I didn't see much point of this feature because firebenders didn't need it and firejet had its purpose. Although, I will say this. The nature of elements are not set in stone. Bending is an art, and an art goes through development over time. Whether it be in the series or in the plug-in, the bending arts are changing with time - through progression. That is the nature of human development and what they practise, in this case, bending practitioners. Over time we have witnessed firebenders revealed knew ways to utilise their bending; they can be advancing with old ways by unlocking variations of commonly practised techniques or they find a different way to use it altogether.
In this instance, firejet has been used on many different scales. Different firebenders have shown how to utilise firejet under different conditions, and I believe this feature pertains to that. Given the nature of firejet, levitation is completely feasible with the right degree of control and limitations, and I think players should have the opportunity to experiment with how to they can control it to the best effect.
As time goes by the way the elements are bent will slowly change. It will open up new bending styles and broaden the usefulness and effect the elements can have in different situations. In the end I feel things like this are necessary. It's important for innovative methods to be born from new, logical features that will help shape the elements and bending in a healthy way. If we do not have that kind of pattern then they way we bend will eventually become mundane and predictable, and that is not how bending works. The elements are vast and that should be acknowledged.
Firejet levitation makes enough sense to be implemented for a lot of players, and that seems fair. I think this will help open up new opportunities for firebenders in a controlled and sensible way. It is a minor feature that, if implemented well (which I trust it will be) will not upset the balance, but simply allow fire to grow and show how creative and skilled players can put it to good use.