I would call a plumber and get their advice and quote on the replacement. It's a pretty big deal, to answer your question, to have it replaced since the materials are not dependable and deteriorate over time. I would check on the cost of replacement and then see if it's still worth buying the home.
I kinow this is an older post and I just came across it and wanted to chime in in case you have not gotten an answer. Poybutylene plumbing has been around for a while and years ago was not an issue with insurance companies. As some componets age we discover defects associated with them. This type of plumbing is prone to leaks. Some installations may last forever while some fail and cause massive damage resulting in huge claims. Insurance Companies use data like this to reduce their risk. From what I can tell it is not the tubing itself that fails but the connectors that may be defective or not properly installed. Hope this helps... more