This is probably not a big problem for most users of session replication. 
However for us it was worth fixing.

In our environment we commonly perform rolling software upgrades across our web
site.  While testing replication, we noticed that as we shutdown and restarted
nodes, the session counts would continually grow.  Basically, we found that as
sessions are transfered at startup time, the local access time is set to the
transfer time.  Code comments imply that this is done to prevent incorrect node
time synchronization problems.  However, it also simulates activity for old
sessions that are about to expire.  Under load, if you restart nodes enough,
your session count will keep growing and growing.

We changed the current behavior to keep existing session access times by
commenting out the call to session.access() in DeltaManager.deserializeSessions
(line 782 in svn revision 548361)