After enabling any one role, all products have to be updated

After enabling any one role for optional pricing, UC starts reporting zero price for any product when the current user has that role. That happens due to the following lines in uc_price_per_role.module, function uc_price_per_role_find_price:

  foreach ($weights as $rid => $weight) {
    if (isset($user->roles[$rid]) && $enabled[$rid] && isset($prices[$rid])) {
      return $prices[$rid];
    }
  }

$prices[$rid] equals "0.00" for any product/role if the price was not manually set. So isset($prices[$rid]) is always true, and the price for the first enabled role is always returned - even if it's zero.
To fix that, you just need to change IF conditions to the following:

  foreach ($weights as $rid => $weight) {
    if (isset($user->roles[$rid]) && $enabled[$rid] && (float)$prices[$rid]) {
      return $prices[$rid];
    }
  }

Now the first available price would be returned, and FALSE if none is available, - as it's supposed to be by design.