Saying that you have unlimited paid leave when that is obviously untrue and leaves the policy open for abuse by both employee and employer. I'm sure the example stating that less leave was taken when it was "unlimited" was because people understood that there were limits but didn't know what the limit was and didn't want to trigger management. Once things are defined, of course people will think that it's ok to take the maximum leave allowed.
I think the guy who quit after two months' leave knew perfectly well it was abuse; he just didn't care. Or maybe he felt slighted by the company in some way (unfair resolution of a conflict, promotion denied, underpaid, whatever) and this was his way of getting even with them.
That means the team was down an engineer for basically an entire quarter, without notice. That wrecks schedules and causes headaches for your coworkers who now have to figure out how to make up for the lost time or figure out what work to cut from the schedule.
2 months is clearly abuse, but I'm worried my CEO or HR will have a far less generous definition of abuse. I'm already reading a thread about 4-6 week vacations where I get at most 2 weeks.