Threepio and Artoo have primary functions (things they were constructed for) but in most of the films they are characters who end up having adventures.

Threepio is a butler/translator and he is equipped for that task.

Artoo is a robot co-pilot/starship repairman.

One of the few bits I really liked in TPM was seeing him doing the job he was designed for, EVA repair work.

Having jets to maneuver into position makes sense just like having a welding torch or a computer interface.

The only time it makes sense for him to use them in the OT is on the sail barge.

Artoo having jets didn't ruin the character.

Artoo laughing his nuts off while Threepio has his memory wiped does.

Artoo as badly rendered CGI getting into oil wrestling with comedy battledroids does.

But conceptually having jets doesn't.

But as I say not my call on PT:R.