For the non-Christmas* vouchers, it's not just that you are forced to spend large amounts each week - the 'spend €50' vouchers are better value because you need one more 'spend' than the number of vouchers you use (and the discount is therefore not 10%).
For example, if you spend €200, you get €20 off the next time you spend €200 - so you have to spend €400 to get €20 off - that's only a 5% return. If you spend your €400 in multiples of €50, you will use a €5 voucher 7 times - so €35 off - or an 8.75% return. The return can never get to 10% because you always have to 'spend' one time more than you get a voucher for.
* The Christmas vouchers are a bit different as they are effectively cash vouchers with no required spend so the discount doesn't depend on 'spend X' rules imposed by Dunnes - but they still don't quite give a 10% discount because you have to spend on something in Dunnes to use them. The best case % discount is about 9% - eg spend €100, get a voucher for €10 - then spend the €10 voucher on exactly €10 of goods - so the discount is €10 on €110 spend or about 9%.