CSS specs are known to take ages to reach the Candidate Recommendation stage which is the stage where they are considered implementation-ready. And even then, a CR can bounce back to WD if problems are found during the implementation and testing stage (this was such a problem with CSS2 that it eventually became CSS2.1).
Since the industry demands fast developments in technology, vendors have to try and implement standards far before they reach maturity. These implementations are often considered experimental and non-final, which brings us back to what you said about them being used for experimental features.
Will the prefixes go away once CSS3 is finalized?
Originally prefixes went away once a module and its implementations were finalized (CSS3 itself is never finalized because it's always growing). These days, measures are being taken to eliminate the use of prefixes for developing technologies almost entirely because they were being misused by both vendors and authors and causing unforeseen but serious interoperability problems; see Chromium/Blink and this CSSWG blog post.