Will there ever be accounting functionality 1) for Simple Invoices?
Simple Invoices is designed to be 1. simple and 2. about invoicing. By adding these features I believe that Simple Invoices will become bloated, lose focus on invoicing, and ultimately become like the crap its fighting against. Though adding an accounting type feature to simple invoices may be useful to some people I believe the benefits of Simple Invoices staying small, simple and invoicing focused outweigh the advantages of such features.
If you run an organisation2) that needs to record income, expenses etc. I suggest using a real accounting software3) to record this type of stuff and on a regular periodic basis journal your invoice income from Simple Invoices into your accounting system.
How I handle my accounts is that I do all my invoicse in Simple Invoices, and do all my accounts(expenses, loans, etc. in GNU Cash) and on a weekly basis I journal my invoice amounts into GNU Cash to get up to date income figures. For info on how to do journals please refer: Simple Invoices -> Accounting system How-to.
As Simple Invoices is free/open-source software and a basic plug in system is being developed, it may be possible in the future that someone adds accounting functionality for Simple Invoices.
GNU Cash is a great accounting program that is free and open source and can handle all your accounting requirements. If you need a real accounting package I suggest you use GNU Cash. For details, see the GNU Cash website: www.gnucash.org.