You can actually calculate the "optimal" strategy for each player. It's pretty straightforward to calculate it in Excel. The 3rd spinner should spin again if he hasn't won already (or tied on $.45 or less). The 2nd spinner should spin again on $.55 or less. The first spinner should spin again on $.65 or less. Of course, actual contestants don't play optimally, plus you get the occasional person who tries to aim the wheel. The odds don't really change for the MDS - the bonus spin is worth more, but qualifying for the showcase is worth more.
Speaking of the MDS, shouldn't someone who just misses the $1,000,000 get more than $5,000 as a consolation for the green section? This has already happened at least twice, and landing on the green section seemed to be a big disappointment.