Nadaltards out in full force to disrupt the thread, NID.
I remember last year a journalist said he considers WTF as high as a slam and asked Fed how proud he was of having both slams and WTF record.
I personally don't rate it as high as a slam but it's definitely the fifth most important tournament of the season for the following reasons (please don't bring the Olympics argument since it's played every four years. in an Olympic year, I rate them the same):
1) it's hard to qualify: you need to be a top 8 (top 10 if you are a replacement) to participate.
2) you have to be at the top since the very first match (unlike slams where you can win 4 matches against much lower ranked opponents)
3) winning it means you are an indoor master, and it's good for measuring one's all-roundness as the OP suggested (unlike the Olympics, where the surface is the same used in other slams, which are a better measure of the player result on a that surface)
4) it has a great tradition and almost all legends of the game won it at least once.