Unfortunately there Is a lot in your iPhone.
So it depends on how far it has cracked and how deep it the crack is.
If you do not have the money to service a repair you could continue to use it but you are risking further damage. There may be a short-circuit in your phone due to the damage. A divided back will also allow regional pressure to different components of your phone so the internals are no longer properly protected.
Vapor, steam or moist in the air will also have more access to your phone.
In short; there is no harm in continuously using a broken iPhone as long as it does not heat up or swell(battery damage). But you are risking the iPhone to break further and be beyond repair.
The back replacement will cost you around $300, If the motherboard receives damage due to the lack of back protection you'll probably pay for a new phone.
My advice: Go to an Apple Genius Bar and ask them your question once more.
If a phone feels slippery; put a case on it.
Even on a $1000 flight you can not go for a walk in the Parc.