Actually, the SRBs do look useful from the standpoint of being "off-the shelf" and our clearly not having $20-30 billion to start over. Perhaps we won't get to some ideal per-launch cost in a bean-counter's imagination, but it wil be good, reliable, comparatively cheaper than current operations by, in round numbers, a billion or so and get us an alternate space-lift capacity pretty darn quickly.
And as for the Heavy-Lift variant, we certainly have a chance of reducing the cost by redesigning the SRBs to be throw-aways, as will be the rest of the booster, ET, and Disposable Orbiter. Not being man-rated will drastically lower all the costs of the HLV.
Without a serious sea-change in launch costs, manned space simply has no point. If all we can ever foresee being able to afford is a couple of dozen people in space at once, then every manned space idea and endeavor should be ended and the money put to much better use.
Really, though, we have the technology to lower launch costs by a factor of 100 or so right now. We don't do it because NASA can't imagine what it would do with the extra capacity and money.
No, not having to bring the shuttle itself back in one piece will just change the cost per pound ratio very much in the right direction. But the mission cost is still high enough to make it an academic exercise. Plus it pushes us in the wrong direction as far as developing sustainable infrastructure.