When client's want to pay there balance on-line, if a payment is unallocated, the balance showing is not reduced by the unallocated payment. Small offices may only worry about the client's total balance as there is usually only one matter per client, and not have any concern for a balance on any particular invoice.
How often would you use this request?
Only when client's use on-line payment which is happening more often. I do not see what the client sees as i don't have a bill to pay on-line.