You raise a very good question about why to own the GP over the MLP. I will give it a stab.
First, I came up with my own metric to compare the MLP's to GP's in a very simplistic manner that does not take into account taxes.
I call it the D/D Multiple or the Dividend to Distribution Multiple.
First, what I do is get a weighted comparison. In other words, if I invested $243,460, I could buy 6580 units(at $37.00) of XTEX and recieve $12896.60 in distributions annually (based on the current $1.96/yr distribution).
Likewise, I could buy 3700 shares of XTXI (at $65.80) and receive $6808.00 a year based on the current $1.84/yr dividend at XTXI. So, the question becomes, why pay more for XTXI to get less per year. The answer is quite simple. The D/D multiple is extremely attractive. Crosstex recently raised its dividend by .12/yr in comparison to a .08/yr increase at XTEX. Remember, XTXI owns the IDR's, so they are in essence a leveraged play on XTEX. So, if you look at my posts on this board, and I have tweaked and refined my own personal model since then, but on a whole, the dividend with pass the distribution on a per share basis, within the next 12 months. On a dollar for dollar basis, it will take probably 2 years before the dividend passes the distribution, but the growth, along with the fact that XTEX is debt free, makes it much more attractive. Also, keep in mind that the GP receives a higher multiple in terms of valuation than the MLP due to its leverage.