While avoiding deform keys for meshes is generally better, how bad it is to use them varies. This page lists the reasons:
Keys - Spine User Guide: Deform keys
The worst case is a mesh that has weights for many bones: each deform key will store a vertex position for every bone that affects each vertex. This can add up to a LOT of data. Other than that, the next best reason to avoid deform keys is that they can be difficult to change. The third best reason is reuse: using bones and weights allows you to use have a different mesh be affected by those bones.
Still, if you need them, you can go ahead and use them sparingly. If you find you need them a lot, you are probably better off setting up a bone to control the deform using weights.
