I use one all of the time. The friction loss of a somewhat narrowed passage over such a short distance is negligable. And since the metal elbow is smooth, compared to the inside of a rubber hose, it's even less of an impact.
The elbow takes strain off of the connection and the hose. It also keeps the hose close to the side of the TT, minimzing opportunity for someone to brush against it, and damage it.
2014 RAM 2500 Laramie 4x4 Quad Cab w/Cummins Turbo Diesel, 2009 Rockwood 2607, and 3 'yaks!
300W of Suntech Solar, a Rogue MPT-3024 Controller, and a Xantrex PW2000 Inverter