Well, I never even considered buying stuff, although I guess Great halls and labs would never be a dead card in such a deck. What would for example, more labs do?
You can eventually add even more Throne Rooms and Monuments, generating gradually more and more points per turn. Or you could safely buy Provinces to eventually end the game.
The reason why I didn't do that: I then increase the possibility I have a hand with (say) chapel, 3 throne rooms and a monument. Keeping it with the 5 I had, I was sure I had 4 VP's every turn.
Adding a laboratory or great hall will not reduce the chance of getting those 4VPs each turn. Adding a great hall each turn makes it 5 VP a turn.
Once you have labs and great halls in your deck the risks of an extra throne room+monument causing a bad draw become small.
Adding a graverobber will allow the trashing of the chapel to gain a lab so it quickly repays itself and gives many more options going forward.
In terms of high level play, there's also the concept of threat. If your deck threatens playing a throne room on a graverobber, trashing two labs for provinces, then that threat can cause your opponent problems. That threat doesn't have to be certain and it doesn't have to be realized, but as long as the threat is there the threat might be useful.
