What are Y5V or Z5U capacitors good for?
I would say the uses can vary depending on your initial project goals/specifications (e.g. what temperature range you want the circuit to perform under, voltage range etc)
You decide the specs/tolerance limits for a particular project, so if you run the numbers and  the circuit will function under worst case scenario with slacker tolerance on certain components then all should be well.
This may mean in one project you avoid them completely and in another you use nothing but.
Generally I would agree that they are commonly used as cheap decoupling/bulk capacitance, but there is no reason why you couldn't use them for e.g. a rough timer/oscillator if it still works as intended within your specifications.
Check the datasheet for graphs over temperature, frequency, voltage, etc and decide whether the part will be suitable for a particular use.
Monte Carlo SPICE analysis is a useful tool for determining how a circuit will perform with component variations.