Joint assessment can be an advantage where one partner has earnings marginally into the 40% band and the other partner has a lot of earnings at 40%.
If the lower earning partner had for instance, 2000 euro taxed at 40 % and made pension contributions for 10000 per year. If individually assessed they would only get tax relief at 40% on 2000 euro and 20% on 8000 euro. If jointly assessed they could potentially get tax relief at 40% on the whole pension contributions of 10000 euro. The pension tax relief is always applied at the couple's marginal tax rate.