Yeah, it would give you some exemption, although it's a negligible amount. I personally regard PPE as being a fantastic course, very well rounded and it offers a good broad-based education in the social sciences. Also, it's one of the most famous degrees in the world.
As for working in IB, well...yes, it would suitable. However, it's not going to gain you automatic entry (nothing will, of course). You need to do a huge amount in your spare time, and attempt to impress them with that too.
For Banking/Accounting, they generally seem to love people who've studied Accounting and Finance (esp. at LSE/Warwick/Bristol). If you're certain about a career in Finance, Banking etc., then you may want to consider this avenue. It will provide you with a far higher number of exemptions (same will a single honours Econ. degree).
However, if you're looking for a more academic education, then PPE is perfect. It combines three of the most respected social sciences, so you obtain a variety of skills. Whether you're looking for commercial knoweldge and a exemption from much of the ACCA/CIMA course, or a well rounded broad based education should dictate your choice. I would recommend the latter.
You will not be 'better off' with either. It's not that simple. There will be less work for you to do after graduation with an Accounting/Fin. or Econ. degree. However, that may not necessarily mean you're 'better off'.
J.S.