Holy water can be good if you see a couple of undead huddled close together. But yes, you have to be aware of such opportunities in the heat of the fight. It's the same with coatings. If you notice there are a lot of enemies with a particular weakness (e.g. no poison resistance or low CON), it could be smart to take the time to pick an applicable coating. And then also abandon the idea of handling one enemy at a time. It could make sense to have someone weaken the entire herd first. Consumables can enhance creative combat.
There are plenty of fights that you can see coming, and have time to prepare for. Picking the right coatings and preparing bombs and potions can make those fights more fun. It helps to go into turn based mode for that, just before initiating a fight, because of the turn timer that some effects have.
Sometimes the consumables will have little effect. But by trying them anyway you get to learn about them, and hopefully remember using them when good opportunities present themselves.
