I think this is a great topic.
There are 2 parts to the answer for me. The first is the value of a game. I am dying for them to release a Stars hardtop as I find it hard to put a $900 playfield in a game I bought for $200.
The second is the condition of the playfield you have. I agree that sometimes it is best to leave an original and play it. But in my Stars for example the ball does crazy stuff that makes it frustrating to play sometimes. That playfield is worth nothing except for wall art so why wouldn't you get some value out of it and give the hardtop a shot.
I am working on my 11th total restore on an EBD project that is a mess. I planned to put in a CPR playfield but thought what the hell, try the hardtop (my first) and see what happens. Again the playfield is worth nothing from a pinball standpoint so I figured to try and see.
The big surprise for me was the hardtop was much much much easier than a playfield swap especially on games with both switched illumination bus wiring AND drop targets which can be a bitch to get adjusted just right for a new playfield. I totally rebuilt every mech but they screwed in exactly right every time with no need for adjustment.
My $.02