As long as you clearly document that the incoming data is going to be modified, it's not a problem. And in a lot of cases, the data either comes from the network or is read from the file - so the buffer is going to be discarded at the end anyway... why not reuse it?
And yes, today it would be easier to make a copy of the data... but remember we are talking about 90's, where RAM is measured in megabytes and your L1 cache may be only 8KB or so.
And yes, today it would be easier to make a copy of the data... but remember we are talking about 90's, where RAM is measured in megabytes and your L1 cache may be only 8KB or so.