The Memotron's samples are also looped. I have always believed that the sample should be as close to the original as possible, and if the user wants to loop them, then they can do it from their own sampler. But they should have a choice.
The only looped samples I sell are for the Hammond B3, Vox Continental, and Gibson G101 organs. As they were all oscillator or tone wheel sources, looping is not such a big deal. B3's do have an "attack" when they key is initially pressed down as the key hits the contacts (a.k.a. Key Click), which is a good quality of the organ. I simulate this within the Motif with an attack parameter.
Mellotrons, however, replicate the actual bow attack (or other instruments) that cannot be simulated with a high degree of accuracy. So I question looping these sounds in the first place.
The Pinder CD is not looped, and is completely authentic. I highly recommend it.