Hawk Diesel
Adventurer
Well, one reason I can think of to include "good" monsters is because only the DM should be able to decide what a "good" monster is. Also, I'm glad that WOTC gives me the power decide what and how I use these things, rather than prescribing what is "appropriate" by only including "evil" creatures.
Additionally, the monster manual presents both lore and stat blocks. While WOTC has presented them as linked via their proximity in their presentation within the book does not mean they are inherently related. At anytime I could swap the planetar stat block and skin it as a hobgoblin, or take a Medusa and substitute the beholder's stats. In this way, the monster manual is more like a set of Legos than fixed toys. Yea, you could follow the instructions to create a Lego X-wing, but you could also go rogue and create anything with those pieces. The same goes for the tools presented in the monster manual.
Additionally, the monster manual presents both lore and stat blocks. While WOTC has presented them as linked via their proximity in their presentation within the book does not mean they are inherently related. At anytime I could swap the planetar stat block and skin it as a hobgoblin, or take a Medusa and substitute the beholder's stats. In this way, the monster manual is more like a set of Legos than fixed toys. Yea, you could follow the instructions to create a Lego X-wing, but you could also go rogue and create anything with those pieces. The same goes for the tools presented in the monster manual.