Personally, I feel that a well structured pre-launch containing phases of usable, quality product related content to warm up prospects a definite conversion booster over hitting them cold on launch day ... problem is that list owners are not as willing to 'give up' their subscribers to the pre-launch list of another merchant as they have in the past.
Much of that is due to list owners feeling they are getting burned by less than quality products and launches, causing them to lose favor with their list in the process ... so they will tend to be choosy, and you can't blame them.
However ... if you've done the proper testing inhouse and/or with a few of your top partners and are getting solid results, I see no reason not to employ a pre-launch ... even if it's only a few days.