pt, Since every dollar you put into an IRA is deductible from your ordinary income, every dollar you withdraw from an IRA is taxable as ordinary income, never as capital gains. The principal value of IRAs is this ordinary income tax deferral feature--you get a deduction today for moving your money from one of your pockets to another and you don't owe any tax until you return it to the original pocket.
IRAs do not allow you to convert ordinary income, which is taxed at higher rates, into capital gains, which are taxed at lower rates. Thus, many people pack their IRAs with high dividend stocks that may have less prospect of generating a capital gain. I have stocks like NLY, D and EXC in my IRA, but not AAPL, which is in my taxable account.
MLPs do not pay income taxes, they make distributions out of their distributable cash flow (DCF), which is not the same as net income (see Alerian's MLP Primer to understand this essential difference). Typically, a significant percent of those distributions is a return of capital, since depreciation is one of the elements added back to arrive at DCF. Although they reduce your taxable basis, returns of capital are not taxable, so from a tax perspective, the last place you want to put an MLP is in your IRA. MWE has good prospects of appreciation, so that is one more reason to keep it in a taxable account--all that appreciation would be taxed at ordinary income tax rates when you withdrew it.
Purely from an income tax perspective, there are other reasons to avoid putting MLPs in an IRA, and no good tax reasons for doing so.