I think its a combination of getting a degree that requires a skill but not getting too specialized unless its in a booming growth sector...A good example is engineering/sciences...
Anyone who has a bachelor's in ME can get a job in any field practically because they are so much in demand but if you got a PhD in ME then available jobs become restrictive just because there aren't too many out there due to pay/need...
A specialized PhD in Biotechology is definitely worth it though since those folks get sucked up by Pharmaceutical companies like there's no tomorrow whereas a bachelors in Biotechology may get a job but after quite a bit of hunt and not much pay...