Of course, if a park were not to turn to the original manufacturers in a situation where they would need a part, they could void warranty (Slammer is not an issue as S&S realised how much of a headache design it is)...
And of course, just because these external replacement parts are cheaper, doesn't mean it doesn't result in a bodge job... And cause massive amount of difference to how the ride runs (See Slammer's handbook covered in post-its and appendices)...
Personally, I would not condone a park to just "go for cheap" unless it was the only viable option to getting a ride up and running, but as we can see with Slammer, this in turn can cause just as many problems... Because these universal components may not be designed to cope with the excessive work done by theme park rides (the state of Samurai is a good example here, as it was never meant to be a permenant attraction in the original design process, as such, after years of operating a high levels at Chessie, it was effectively buggered for the rest of its working life)...