My take on this is, if you ONLY use your arranger in LH chord, RH solo mode, yes, you can probably get away with 61 notes, But arrangers are capable of so much more that just this, and for that, 76 is just about right..... Less weight than an 88, and bottom E for the left hand bass players, although 73 C-C is OK, as long as you have the Upper parts shifted up an octave. Enough range to play pretty pianistically on top of an SMF or in a band situation, and just about enough notes to squeeze in a third part in a pinch.

Just don't forget, many, many users of arrangers don't want to be restricted to HAVING to use them in arranger mode ONLY..........