In general the more disks (or more specifically the more disk arms) you have in a system, the better.
Yes, Martin is correct that the performance will only be better because of single level storage, the more access arms for the IBM i to spread the information across and retrieve the better the overall performance.
There is never a down side to adding more disk especially if you have put it in the same ASP as your other disk.
You could add it to the system for segregating say production from test libraries to be able to track the disk usage per ASP (Auxillary Storage Pool) or put the journal receivers in a spearate ASP but the additional work to keep the files and programs where you want them is a lot more hands on managing. Best to leave them all in the same ASP for performance reasons.
Both the answers above are correct but you might also like to consider which type of disk IOP you run the disks under. The newer ones have bigger cache and this can make a difference to performance. Also, 70Gb disks can provide better performance than the 140Gb ones as both disks have the same number of arms but obviously the 70Gb’s have to retrieve less data.
Hope this helps,