The reason ICJ and ICC should be careful about giving decisions they cannot enforce is because over time it can undermine their credibility. Eventually you end up with these international bodies writing the equivalence of "open letters", which is fine I guess... but if those actions have no effect on outcomes it really starts to undermine the Institutionalist side of IR theory.

Personally, I find that unfortunate. While I think the realists are mostly right, it is helpful to have international legal institutions that maintain some degree of legitimacy and power for the purpose of norm-setting.