The company is sending "Welcome Back" packets, and banking on nostalgia to retrieve some of the enormous talent it has lost over the years. Every company wants talent. But why would you want to fill your company with returnees?
A whole cohort of veterans can be tricky. Just imagine the litany of phrases like "in the good old days" or "the way we used to do this" or "in my day." Remembering the grand old past is no way to create a shiny new future. Now that the company is in serious trouble, harkening back can do little beyond remind everyone how much has been lost.
And recruiting them into the mess that Yahoo currently is smacks of desperation. Does Yahoo really want to bring back the people who created the mess in the first place? Why don't newbies want to join now instead?
This doesn't mean you should never take people back. People who leave aren't traitors. They just want something different, at least for a little while.
Indeed, there are circumstances in which employees leave in order to gain wider experience of a kind a current employee just can't provide. Many companies eagerly recruit parents who left to have time with their kids. When Volvo ran an experiment, in which it required men to take parental leave, it found that the guys came back with better communication skills, and they were more organized. So career changes and breaks can make everyone more productive; time away could mean they have more to offer now.