I was using the military uniform as an example of how His Excellency (like his immediate predecessor) has demonstrated that he is willing to take a few extra steps to remind Canadians of who and what he represents. Wearing decorations would, I believe, be helpful in this regard, even if Canada Day is a comparatively informal occasion in actual practice.
The Queen doesn't need to wear decorations to remind people that she is our head of state; after all, her face is everywhere, from coins to official portraits in public libraries. Not everyone knows who or what the GG represents though (especially if their understanding of the position has been negatively affected by the way people like Peter Mansbridge explain it: "oh, so the Prime Minister appointed him?" sneers the cynical pundit. "He must be politically-connected buddy of Stephen Harper.")
Wearing nothing but a business suit at official events makes the GG blend in with other politicians, who are also wearing business suits; the GG should be the centre of attention at Ottawa's Canada Day celebrations, in lieu of the monarch herself. The business suit also makes him look like one of those "ceremonial" presidents in the European-style parliamentary republics that republicans have in mind as a replacement for our constitutional monarchy.