This dilution idea is a VERY OLD discussion. How does a company that is required to pay out its "free cash" obtain cash to expand existing properties or build/buy new ones? They have 2 options - debt or equity. I'm not aware of any other choices. You can do it all with debt except the bond holders require security. The security they want is equity so MLPs must issue additional equity.
The key is how the funds are used! It the expansion or addition produces more distributable cash flow than the cost of the equity and debt issued for that cash it is accretive to existing unitholders.
Would imagine with the very high yields of MLPs we will get lots of these comments in the coming months by those who do not know a MLP from a REIT. Hope this answers the question.