The older versions of Maximo, up until version 5.2, had less flexibility to add custom fields, so extra fields were included for that purpose. Take note, though, that some of the fields also had automatic crossovers built in, i.e. an extra field on the asset record, for example, would automatically cross over to the related work orders. The old System Admin guides have a list of those crossovers / field mappings.
In later versions those fields fell into disuse because of the ability to add your own field, but I suspect that a good portion of the crossovers are still active in the code.
With that in mind, there's no reason why you couldn't use the old extras. I did so myself recently since I needed a small, quick change and didn't want to have to schedule an outage.