Hi,
It depends on client requirement, whether to go for +ve or -ve. Even with bio-metric, you can go with -ve if client not interested to track time ins and time outs and their validations.
Majot advantage of +ve time management, is it tracks all absence data by default if there is no attendace maintained.
for more info see
http://saphroutlook.blogspot.com/2009/03/positive-negative-time-management_25.html
https://scn.sap.com/thread/2096027
Regards
S.Ravi