A QS report isn't necessary but you should always ask your accountant if it would be worth it. If you have a house built in 1960 with little or no additions, chances are your QS report will be worthless. If you have a unit in a highrise apartment building with a body corporate that was built last year, I would strongly recommend one.
If you don't use a QS, you'd be having to make a guess on what you could depreciate for your accountant, because chances are, you won't be hiring him to inspect your property for depreciable items. That can be quite difficult without invoices. I don't want to even know how a person can establish building cost unless they either do some major research on the builder's invoices or throw darts at a dartboard with an assortment of numbers on it.
As for a report from another unit, that won't hold for you. Your unit may be larger or smaller, more expensive or cheaper, contain more items or less, etc, etc. In addition, the QS will not help you out if the ATO comes knocking. If you have a report, you could approach the QS for giving you a lower fee since they already did a lot of work surveying the original unit and it may just require a small inspection and a slight modification to the report you currently hold (Ah, as Pete said).
I haven't heard the ATO say anything about not challenging qualified QS reports. I doubt he would give that assurance. Acording to a report I read a while ago, a high number of QS reports when audited were reviewed down. That represents a revenue raiser for the ATO.
But if you are going to get a QS report done, make sure it is done by a professional. There are too many reports getting done by dodgy operators who won't be around when the taxman cometh. Some of these people use rates from 4 years ago and haven't updated them.
A QS report is useful as you know you are squeezing your property for every depreciable dollar claim you can make within the tax office guidelines. Thats why I get them done.