Assuming your gaining the same % of expertise as hit % your losing, your threat will not diminish.
You will have more difficulty landing taunts, (you can glyph for it when necessary) demo shouts, thunderclaps, shockwaves, shield bashes, and heroic throws as these skills are not effected by expertise. However many of the debuffs or interrupts associated with these effects can be provided by other raiders.
See Venes Blog
here on the subject. If given the choice between 1% expertise and 1% hit (after the exp. soft cap) 99%+ of the time I will choose the expertise because expertise also lowers your incoming damge.