I will incline towards Uncle Dave's perception.
The machine is great at repetition, and even with longer loops, it still repeats the same thing over and over again.
I have no desire to get overly technical, but just this once, I will say that if the longer loops enable a persons short term memory into being fooled that there is actually some improvisational playing happening or "live/realistic" playing then all the power to you.
But even with longer loops there is still repetition, and if you get bored with 2 bar loops, then soon your brain is going to get used to the longer ones (resulting in boredom), and then more and more longer ones, and the cycle will end only when you have a real human player to surprise you at every musical turn.
I personally am happy with the strength of the machine, which is reliability in repetition. Unlike a human player, a machine never tires of repetition. Keeping the rhythms and styles simple in yammie boards has been a major decision by Yamaha, I believe.
Korg, pa80 for example, takes the route to satisfy the longer loopsters. In putting longer loops, korg has taken a big risk, because if the longer loops don't have satisfactory fillings, then the whole loop is useless, ofcourse, you could always turn off the track causing the annoyance, but what if it was the rhythm track and the filling was part of the main rhythm track? You couldn't turn that off could you.
But with yammie, I don't have to worry about any of their loops trying to be "all that". I concentrate on myself, and what i can offer to enhance the simple mix.
Just like the psr2k screen, i believe longer loops detract. I like yammie arrangers because they do a simple thing perfectly. If i wanted realism/complications I would join a band.