does it give a better rate of fire than a regular 9.6 battery or is it a waste of money buying a 7.4?
Depends on a lot of stuff. It will usually help over a 9.6 mini in terms of final rof, but your trigger response MIGHT be slightly reduced, though this is an extremely rare circumstance (Going from a really nice nimh to a really poorly planned lipo purchase)
I would recommend that you might do a little research on lipos, there's a boatload of information about them on here, arnies, airsoftforum, etc.