“I’d like it very much if all the primaries were held on the same day.”
All that would do would make the candidate with the most name recognition win and give even more power to the national media to pick our candidate.
No thanks.
That would assure that the candidate who had the most positive media expose prior to primary day would win. Thus, John McCain would still have won under your proposed rules if we could turn back the clock, and Mitt Romney would likely win in 2012 if we could jump ahead. McCain wasn't electable, and Romney probably isn't, either. I will tell you right now that no one I know would vote for him, come.
Also, if later in the process the candidate turned out to be a complete moron (take your pick for examples) or some scandal broke (I'm looking at you, John Edwards), we'd be stuck with an unelectable mess, and a long, embarrassing, expensive mess leading to election day failure.
Make them prove their worth over tim,e and the American public can decide if they have what it takes.