Not in current implementation. As the merge return value in used in CoreBundle to properly add items to order.
In first version of this PR, it was throwing an exception (@784011f72a671c462fa677c4b37494d7ee9adf35) and returning no value.
That's also why I proposed #855. See my last comment about the issue (https://github.com/Sylius/Sylius/issues/855#issuecomment-32598542)

To avoid too many checks, maybe I should add a parameter to merge whether to check if mergeable or not.
```php
/**
 * Merge other item with current item
 * @param OrderItemInterface $item
 * @param Boolean $checkBeforeMerge
 * @return $this
 */
public function merge(OrderItemInterface $item, $checkBeforeMerge = true)
{
  if ($item === $this) {
    return;
  }

  if ($checkBeforeMerge && !$item->equals($this)) {
    throw new \LogicException('Given item could not be merged');
  }

  $this->quantity += $item->getQuantity();
}
```

@stloyd @pjedrzejewski What do you think?