This is explanation is given without Prejudice. It's assignment of the parking lot to your unit. They missed it out in the original S & P agreement, they now have to rectify this before handover. They are also 'slipping in' the amendments to the Garbage disposal unit and the Library which will now be redesignated to a reading room. Cost cutting exercise to both items! Manned Library to reading room seems OK to me. But the Garbage disposal issue, you have to come to terms with this, i.e. whether you really want to "pick a fight" with them, because it will come down to it. The developer is not budging on this or the compensation idea!
Downside to not signing the Supplementary Agreement, is your unit will not be alloted a specific parking lot and it will fall under "common property" so you will have to find an avail. Parking lot when you need to park. The developer will now not impose extra maintainence fee for the alloted car park lot as it already falls under the maintainence charged in Schedule 2.
Please note, you need to do this without delay as when the developer gets approval from the Authorities on the title to be issued, those that haven't signed the Supplementary Agreement will need to go through the submission process themselves - the above was related to me by the sales person at Puteri Harbour and also the lawyer I spoke to.
Hope this helps. Best seek legal advise if still not happy with situation or have doubts