One good reason not to have a playoff is...
...the whole season is a playoff. In most cases, one loss kills a chance at the title. Plus a large playoff significantly reduces the significance of the regular season, which I definitely do not want.
However, with that being said, your system actually is a good one, talisitider, if we are going to have a playoff. You only allow conference champs into the playoff. No "at-large" bids means no arguing over who deserves them. Basically, go win your conference, then let's talk about national titles.
A large playoff like you described has always been my "other" preference. It would involve changes so significant that I do not think we will ever see it.
In fact, I worked up a sample of what I might look like. There are 8 conferences with 8 teams each. Every team plays all 7 conference opponents each year. Each team gets 4 non-conference games in which they can preserve the rivalries which inadvertently get disrupted by my new conference affiliations. There are no polls (or if there are, they mean nothing in the postseaon). Everyone plays 11 regular season games. There are no conference championship games. Everyone plays everyone, so conference titles are won on the field, all season long. The 8-team playoff takes 3 weeks. The 4 games played the first week are called quarterfinals, NOT BOWL GAMES. The 2 games the next week are called semifinals, or the final 4, NOT BOWL GAMES. The last game the next week is called the finals, or the national championship game, NOT A BOWL GAME. The two teams that play in the finals only play 14 games.
Here are the conferences:
ACC - FSU, Miami, Georgia, Georgia Tech, S. Carolina, Clemson, NCState, UNC
BIG 12 - Oklahoma, OSU, UT, TAMU, TTU, Missouri, Arkansas, LSU
Big East - Virginia, VT, West Virginia, Marshall, Maryland, B.C., Pitt, Syracuse
BIG TEN - Mich, MSU, Minnesota, Wisconsin, Iowa, ISU, Purdue, Northwestern
MAC - Miami, Toledo, B.G., NIU, Louisville, Notre Dame, Ohio St. Penn. St.
SEC - Alabama, Auburn, Ole Miss, Miss. St., USM, Memphis, UT, Florida
PAC 10 - USC, UCLA, Cal, Stanford, Washington, ASU, Oregon, OSU
WAC - Boise St., UTEP, Fresno St., Utah, BYU, Colorado, Kansas St., Nebraska
In general, my conferences are very geometrically centered - much more so than what we currently have. Some rivalries are separated, but they can still schedule each other as OCC opponents, if they choose to. The only conference I think would be really upset is the BIG TEN. They went from a powerhouse conference to a very average conference. Michigan would win it most every season.
Anyway, it ain't gonna happen. You cannot break up the power conferences like that.
Also, what happens to the other teams? This only includes 64 teams that I hand-picked. I actually have a system to handle that too, but again, it could not be implemented. Too many schools would be ticked-off for it too ever work.
-Sully