G12 user... been using adapters from Lensmate instead of Canon as it gives no vignetting...

http://www.lensmateonline.com/store/g10.html

I do think that adding a ability to accept filters for G12 is an added advantage for shooting... you can add Polarizer which is a very nice and useful filter when shooting under bright sun light. Other than that, you can add in all sorts of conversion lens such as tele-converter, macro magnification, ND filter and etc... it gives you more creativity to compose and create that lovely shot...

However, this is really depends on individual, as you need some bucks to get all those equipment. Some of the effect you can actually get it from photoshop... so, it's up to you to decide whether to do heavy photoshop to get the effect you one later, or get the effect with the accessories before hand.