In the Age of Obama, it's become customary to look at a "big speech" as an epochal event.
Romney has a lot going for him -- a history of accomplishment in the public and private sectors, a depth of economic understanding that puts the President to shame, a deep network of significant donors (which would allow him to run a long, vigorous campaign and ultimately help him compete with President Obama), a strong family life, etc., etc.
But the health care reform he supported in Massachusetts is an Achilles heel that threatens to cripple his entire campaign unless he can distinguish it from President Obama's (more effectively than in previous efforts) -- or, better yet, communicate that it has served as a test tube that proves that government-driven "reform" won't work in the health care market.
Making the speech is a risky move for Romney, but a necessary one. Without a more effective way of addressing Massachusetts health care, he runs a very great risk of never even reaching a significant core of GOP voters -- whose support and enthusiasm he would need, if not in the primaries (among a fragmented field) than certainly in the general.