When you have a spawner, and a spawn egg, you can change that spawner into any type of mob spawner. A lot of the times, people don't know about the basic spawning requirements for commonly used spawn eggs on spawners. I think that when placing a spawner down there should be a little pop-up warning menu that tells you that you should understand the spawning requirements for the type of spawner you are creating. The most commonly used ones should be listed and have the basic requirements for them, not all, but the commonly used spawn eggs. E.g squid, slime, and so on. This would be really helpful for players as it would give them a warning to understand that basic requirements still apply to spawners.