Problem is, the mounting tabs are on the front of the frame tube close to the spring and so if you simply bolt the 'P' clips on them, then the whole thing sits too far forwards and would foul the spring, plus the reservoir is too long to fit inside the 'cradle' of the frame where it would be anyway.
As such, you'd need to extend the 'P' clips significantly backwards to get it further away, at which point it'd be dancing around on the long extensions unless they were really substantial, so a bracket works better IMO.
Plus, it's not a lot of weight, especially mine which I made from fairly thin stainless (don't know how thick the OEM part is?).
You could always make a Carbon Fibre bracket...
