"What prior losses?," you ask. Most MLP's generate an ordinary loss on the K-1 for the investor. Mostly due to depreciation being a non-cash deductible expense. I guess its true what they are saying. That you can't deduct any of these suspended losses until the MLP generates ordinary income for you from operations, or you sell your entire investment in the MLP. Maybe substantially all of the investment, no pro-rata, I don't know for sure. I haven't faced the issue.
But it seems to me that your lifetime cash in, cash out tally with respect to the MLP will tell the whole story. Assume one invested $1,000 cash in an MLP, received $500 in cash disrtibutions during the time they held the investment, and sold the investment for $1,500 cash. They made $1,000 on the round trip. The only unknown is what portion of the $1,000 is ordinary income and what portion is capital gain.
I don't see how keeping meticulous records of your suspended losses is going to do anything magic for the investor. A big portion of that $1,000 gain is going to be ordinary due to all those ordinary losses that the MLP generated. So call it all ordinary and avoid the tracking headache and the CPA bills.
If one is in a low enough tax bracket, what difference does it make if they error a little too much on the side of ordinary income? Choose some number that is reasonable. Try to figure it out from the shoe box full of K-1's that you've kept. But a sure way to diminish the yield on your MLP investment is to engage a CPA. We are in this for yield, afterall.
Apart from needing to sell all to release the passive losses, the tax handling is complex for a partial sale and Turbotax will not handle it without manual adjustment beyond the ability of most TT users. So there are at least 2 good reasons to sell all or sell none.