i have horrible visions of a bit of pipe thats been crushed in a vice slashed at an angel and gobed on with a bit of weld 
steel can be a PITA to work with espectaly with what you'll be trying to do with it
Thats pretty much what Ive done for fitting some throttle bodies and steel would have been much easier as it doesnt need anealing to stop it cracking as you crush it to the required shape. You can still do a good job with this method (and lots of people do it this way) with the alternatives to produce a "nice" result being a lot more work/expensive to make (casting, machining from solid, composites, formed and welded sheet).
As for the other reasons why you shouldnt use steel that have been mentioned here, what a load of rubbish. Ali conducts heat much much faster than steel ever will, can be a bugger to form (it cracks when forming if not carefull), is a git to weld as a rule, is more expensive to buy and, like steel, even ali needs some form of corrosion protection to stop it from crumbling in front of you. The only real advantages ali has in this situation is that its a bit easier to machine and that its lower in weight which even then you could get that down using steel as you can use much thinner material for the same strength.
