Until a few days ago Malaysia's position was the plane had crashed, then it abruptly switched to the position that the plane had passed through Thai airspace and gone on somewhere else based on their own radar.
So I can see from the Thai point of view that if they had any radar data it went from being irrelevant to being redundant.
Nobody thinks the plane crashed there so it may well be redundant. We don't know if it will add anything. Even if it provides clearer readings of the plane's location and altitude it is unlikely to provide any insight into the critical questions.