It depends on the needs of your client. Depending on the complexity of your organization, it could be interesting to have such a Sandbox or not.
If you just want to test some bulk loads, or similar. You can get away with just a Partial Copy since you can load as many records as you want even though you the docs say you can't. The main issue with a Partial Copy sandbox is that the record-copying algorithm is broken and you will find yourself with inconsistent data on many occasions (Like having a Campaign Member without Contact/Lead).
On the other hand, if you need to actually test functionality against the whole dataset your client has, then you will need such a Sandbox. It's useful for detecting Governor Limit violations. For example, we have a very heavy duplicate management functionality coded with Apex in our organization and if we had not run it in such a Sandbox we would've never got the
CPULimitException until it had reached production.
But just like I said at the start of the answer, it's very dependant on the size and complexity of your org. For 90% of the client's I've worked in, I can say they didn't need one.