So, the leader must renew their lease every (period * ι), where ι < 1. The problem is that this assumes a synchronous network, or reliable clocks - since the Paxos implementation isn't tied to the master -- this results in the problem that you've given up safety, example:

1) The process p1 on the machine holding the lock stalls (VM stall, let's say). 2) τ seconds have gone by on the Cassandra cluster, and the lock is removed 3) Another process, p2 takes the lease 4) The process comes back, only to resume processing, and not realize its lease has expired...

Without the leasing system, and the leaser being tied together, it's very difficult to guarantee safety, and liveness.