From a roster perspective, as in "afraid" he'd make the Lakers better, I find it irrelevant. As much as I liked Rondo here, even as I complained about many of his flaws throughout the years, he's no longer that much of a player... and if he gets PAID like he wants to get PAID, then even better.
From a Celtics fan perspective, I'd be incredibly angry at it. He would be the 2nd player of main contributors of our latest Championship run that I'd have to write-off completely... tarnishing my memories from a championship run. But not really at the level of Ray Allen who did a complete BS move towards us, but it would be a real shame to see a Celtic go voluntarily to play with the Lakers.
The only player I'd forgive if he wanted to play there would be Paul Pierce, he would've earned it, as much as it would hurt to see him in Lakers gold and purple.