lastlog support would allow an administrator to reliably look up the last time a user logged in through ThinLinc with the lastlog tool. The current wtmp-based solution is not as reliable, as the log file is commonly rotated.
OpenSSH seemingly has some code for handling this, and I've found references to a PAM module that takes care of this. Even if OpenSSH manages this, we'd still need to find a way for HTML5 logins to be recorded.
Note that "last" works fine with thinlinc though, so there is information about thinlinc logins on the system, just not in "lastlog".