What a glaze actually is
Glaze is a thin layer of glass melted onto the surface during firing. Glass is chemically inert at the temperatures tea involves and has no open structure for liquid to enter, so a fully glazed interior takes up nothing and gives back nothing. That single fact is what makes a glazed pot usable across a whole shelf of teas and an unglazed one not. It is a property of the coating, not of the clay: a glazed stoneware pot and a glazed porcelain pot behave identically in this respect however different their bodies are.
Three states, not two
Vessels are not simply glazed or unglazed. Fully glazed means inert. Unglazed means porous and dedicated. And partially glazed — a glazed interior with a raw foot ring, a glazed body with an unglazed rim, a thin or interrupted slip — is common and is where care goes wrong, because the piece looks sealed and has an absorbent patch. An unglazed foot ring holds water, marks a stack and can grow mould if a piece is put away damp. Check the inside, the rim and the foot separately before deciding how to handle a pot.
Crazing is a fourth state
A crazed glaze is one whose network of fine cracks reaches the body beneath. Sometimes this is a fault of fit between glaze and clay and sometimes it is designed in, as with crackle ware and much Hagi. Either way the consequence is the same: liquid enters the cracks, the piece stains progressively, and it cannot be restored to how it started. For crackle ware that progression is the reason people buy it. For a pot you wanted neutral, it is a slow surprise.
What unglazed buys and what it costs
It buys the seasoning process and its associated tradition, a particular surface in the hand, and access to a whole world of pots that are not made any other way. It costs flexibility — one pot, one tea — and it costs convenience, because an unglazed pot may never meet detergent and must dry with the lid off. It does not, on any evidence TeaHQ can find, reliably buy a better cup of tea. Buy unglazed because you want the object and the practice, and be suspicious of anyone selling it primarily on flavour.
A practical rule for a mixed shelf
Keep one glazed vessel that goes anywhere and can be washed properly — a porcelain gaiwan is the cheapest version of this and does everything. Add unglazed pots as dedications to teas you have already established you drink regularly. That order means you are never in the position of owning three seasoned pots and no neutral vessel to taste a new tea in, which is the most common shape of a teaware shelf that has grown without a plan.