It is always best to get an appraisal when purchasing distressed properties - short sales, bank owned (reo) - as is a home inspection, whether you are getting financed or a cash buyer.
There are different types of appraisal products you can get, from a simple dest-top market analysis to a full interior/exterior appraisal, and several others inbetween. Unfortunately for many buyers, getting an appraisal is usually the last element acquired in the transaction, therefore the need for a contingency clause in the purchase contract. Investors will always determine the market value before offering or purchasing a property, home buyers would be wise to do the same.
The main objective in a short sale situation is for the property to sell as close to the owed amount as possible to reduce the amount of loss for the lender. And most lenders will already know what the appraised market value is and may not accept an offer at that value and opting to foreclose.
A home inspector will check the property for condition and functional elements that buyers and others most likely will not be aware of. An appraiser will do similar for the market area condition and trends, property marketability conditon and amenities, and other things affecting value, when determining the market value.