Is it necessary to replace a jamb when replacing a door?
It is not worth it.   Based on a ton of factors the hinge position is very very unlikely to line up right.   External doors are all over the place on this and you don't want to be chiseling/routing your jamb and if going steel this isn't an option on the door.   

This doesn't even get into a 34" door is sometime 33 7/8" or that the door is a hair wider or whatever - even if you took measurements and everything lined up on paper I would give you a less than 1 in 5 chance of having a door that fit really good and thats if you can find that.

It just doesn't make sense to use the old one as the new one will assure a perfect fit.   You don't want to have gaps or have a janky exterior door.   The other factor is yes the jambs will take 20-30 mins to put in but dealing with hinge mortises out of position is a complete nightmare - especially when they are just a little out of position.  Sure you can get this to work but will it look good and hold up.   

Exterior doors are night and day in regards to replace.   There is a good reason you seldomly see an exterior door sold without a jamb kit.   

(And I have a very good answer on replacing interior doors - which does not require jamb replacement and is doable in bulk)