Schools like Alabama, Penn State, Notre Dame, UCLA, Michigan, Oklahoma and Texas can get away with always having the same uniform because they are a recognizable brand and a recruit wants to wear that traditional uniform they have seen other great players have success in.
Schools like Purdue, IU and Northwestern who have had minimal football success compared to those top programs and have less brand recognition and exposure can't rely on not competing with the uniform wars now because they will fall behind. Had Purdue never changed our uniforms since the Greise days and people could turn on the TV and instantly recognize that it's Purdue just by looking at the uniforms, than we wouldn't need to be wearing these alternate uniforms. However, Purdue has changed up the uniforms quite a bit since the 60's and
the common fan across the country wouldn't distinguish our normal uniforms from Army or Wake Forest or any other black and gold school.
I personally wish we had that traditional uniform that was never changed and would be recognizable across the country, but we don't so we basically have to compete with other schools by using the alternate uniforms as a marketing/recruiting tool.
I liked the Grey uniforms a lot, and think if they changed the numbers and lettering to old gold and Helmets to black with some gold that they would be excellent.
I think the Bob Greise uniforms (based on photos I've seen) are the best Purdue has ever had. They are simple (like PSU, Alabama, ND) and have the gold I like.