In addition to beachycove's succinct summation I would point out that, as has already been recorded in previous threads in these Forums, most of the older PBs do not begin to recharge an exhausted backup battery before the system battery has reached near-complete terminal voltage (ie, not complete charge, which depends on the battery's condition). This can leave you in a cleft stick, in that if the system battery is NBG, and therefore will not reach full terminal voltage, it may take somewhere from a long time to never to get the backup battery up to scratch. That is, having a fully chargeable system battery is the need, even though the PB may work quite happily from its AC adapter alone.

For intermittent use, and given that you don't care about clock or PRAM settings, uncharged backup and missing or defunct system battery may work perfectly well. For optimal use, you need a working system battery. QED. The backup battery will also not give you part of its designed service: graceful shutdown and maintenance of memory contents for 24hr when the AC adapter (or the system battery) is removed or fails.

de