I will definitely grant you that, but I think it's deserving of more consideration than a blanket statement provides. . . this would not necessarily be geared towards musicians (who already have tools at their disposal), and if the internet has shown us anything, it's that there's always an audience somewhere. Amplitude, for instance, is another musical, rhythm-based game on the PS2 (from the same developer as Guitar Hero, no less) that includes a mode that allows you to remix music tracks within the game interface and upload the results for others to listen to. Strange as it sounds, there was actually a sizeable community based around this simple feature. There are also many people who have spent hours on end using a controller and the basic shapes and tools in the Forza 2 decal editor to create incredibly detailed designs on their cars, so this is definitely something that simply comes down to the existence of a will and a way. . .
As far as usefulness goes, let me draw you an example - and the pun is intended, as my own talents do not lie in the realm of musical composition. . .
To preface, using a game controller to produce music, while possible, would most definitely be time consuming at the least. Any notes generated would need to be heavily manipulated if the user intended them to sound even remotely real - anything less would simply result in the most rudimentary-sounding synth.
Now, what tools do most people associate with visual art? If we're talking about drawings or paintings, the answer will be pencils, paint, brushes, etc. These allow varied applications of pressure, stroke, shade, and texture to create an image that the viewer can identify and connect with.
While I don't think so highly of my own talent as to claim that I created real art, I think it is the method by which I created the attached images that makes them appropriate to this topic. The fireball image was done with Bryce 3d and Photoshop, and just a two button PC mouse. I don't own a graphics tablet. The other was created using a mouse and Windows 3.1 Paintbrush. . . pretty much the CG equivalent of a fistful of crayons, but the best I could muster in 1995.
I've always been particularly attached to these images, because while they were almost unnecessarily difficult to complete (on the futility scale, drawing with a mouse probably rates somewhere between sculpting with a toothpick and trying to black in an entire posterboard with fine point markers), the end result was mostly to my satisfaction - a rare occasion when it came to my artistic endeavors. There were certainly better ways to achieve these, but this is simply the way I approached them. I know I'm not alone, either - I'm sure most folk here have seen the online video of the guy that draws an incredibly detailed car using Windows Paintbrush. Sometimes you just use what you have. With enough work, the first image could have been manipulated to resemble an actual piece of comic book art.
Do I think that the audio software being described will be made available for video game systems? I'm not sure - the cost would be prohibitive, and the work required to make it servicable would likely prevent it from ever happening. But if it is somehow made available, people will use it. Seriously. . . if all the Guitar Hero fans were given a chance to use the same peripheral to try and compose their own tracks? They'd be all over it.
Why draw with a mouse when you could use a pencil? Why use a game controller to play music when you could use a guitar? Why circle the earth in a hot air balloon when you could take a jet, or sail when you could ride a motorboat? I dunno. . .
why not? 