Well, the point is that we could draw on the HELOC and then pay it back to zero a few weeks later. A back-of-the-envelope calculation of expected inflows and outflows suggests we'd pay a fair bit less in interest because we'd only have a balance on the HELOC for a month or two at any given time.
Unfortunately (or fortunately?) we don't have any taxable investment accounts at this point -- we've been able to put everything in tax-advantaged space so far.