I think it is not a €6 monthly topup but instead a €6 per month minimum spend. If you do not spend this, the amount will be debited from your balance.
e.g. if you spend only €4.50 in a month, €1.50 will be debited.
I thought thogh that they were still doing Tarjeta 2. This compensates the €6 minimum spend for a slightly higher daytime rate? I have been meaning to switch to this for some time. Are you sure it no longer exists? It still seems to exist on
http://www.yoigo.com/tarifas/especiales.php