Good question.
Here is one answer I've heard over and over again.
"IIT + IIM gives the best job beta"
And marriage beta too
But getting back to topic, I also still stick by my stance that GMAT/CAT aren't the best yet. I agree with Sei's points that these tests do test your time mgmt, logical skills, etc.. But they do not test your attitude, which is also very important. There are some people with 750+ GMAT, 98%+ CAT scores I wouldnt even want to be around, let alone work with.
Freshers-in-their-new-field(i.e., switchers) are alright. Because they at least know how an organization works. A fresher(i.e., a fresh under-grad) will not know this. Its not essential, but its good to have this experience. Of course, there are exceptions such as those who are part of family businesses, or have done part-time work, or those who can hit the ground running at a new job.
Remember that unless one has prior relevant experience, one will again be tested at the first job post-MBA. Its always an upward climb, and an MBA is not a solution to your career issues. (This rant is because i've seen people joining tier-3 colleges, assuming that an MBA is the golden goose)