People are chasing the yields for a reason...there are few options right now for return of any decent size. Even if they are over valued, most investors are not looking to sell a point or two of gain in the underlying units...they are looking to hold for long term income. That is a difference in the MLP vs a common stock equity.
High income ETF's are having (or are continuing) a good run as well. Muni's, Corporates, Preferred's and Ute's are all going up as mom and dad are looking to replace matured CD's and get out of MMF's that have almost zero yields.
And with commodity prices rising, I don't see MLP's as being richly overvalued.
So, my 2 cents...few sellers, decent returns, few other or better options...the run may continue a while longer.
Traditional valuation for these is a spread to Long Term treasuries and yields are pushing up dramatically. About 300 basis points spread is the historical benchmark and we are trading narrower on many. Keep in mind, these are not risk free securities and the more units they sell, the more risk there is. They are selling units because they are reducing leverage but that puts more operation risk in the unit price. You are focusing on the steep yield curve with the front end yielding nothing, that is short term OK but when the music stops look out.