Because your rewrite return parent::_prepareForm(); which is class Mage_Adminhtml_Block_Promo_Quote_Edit_Tab_Main you will need to rewrite it parent class.
In \app\code\local\Acer\Coupon\Block\Adminhtml\Promo\Quote\Edit\Tab\Main.php:
class Acer_Coupon_Block_Adminhtml_Promo_Quote_Edit_Tab_Main
extends Mage_Adminhtml_Block_Widget_Form
implements Mage_Adminhtml_Block_Widget_Tab_Interface
{
.....
Then copy all the content of Mage_Adminhtml_Block_Promo_Quote_Edit_Tab_Main and then make your changes