One of the comments on the Heroku article mentions this gem, which does just that:

https://github.com/kzk/unicorn-worker-killer

Regarding memory, I think you can do something like this in your Unicorn config:

after_fork do |server, worker| size_in_bytes = 60010241024 # megabytes Process.setrlimit(Process::RLIMIT_AS, size_in_bytes)

We haven't had any issues with memory leaks since we've been using Unicorn and Rails 3. Back in the old days of Mongrel and Rails 2 we had to restart the processes periodically because of this, but we haven't seen any issues with this setup. Here are the stats:

    www-data 21714  0.0  5.6 102928 87164 ?        Sl   Feb26   0:30 unicorn_rails master -c production.conf.rb -E production -D                                                        
    www-data 21762  0.2  8.5 150672 131996 ?       Sl   Feb26   1:58 unicorn_rails worker[0] -c production.conf.rb -E production -D                                                     
    www-data 21768  0.2  8.5 152036 132352 ?       Sl   Feb26   1:58 unicorn_rails worker[1] -c production.conf.rb -E production -D                                                     
    www-data 21774  0.2  8.1 145248 125668 ?       Sl   Feb26   1:56 unicorn_rails worker[2] -c production.conf.rb -E production -D                                                     
    www-data 21780  0.2  8.3 148792 129736 ?       Sl   Feb26   2:01 unicorn_rails worker[3] -c production.conf.rb -E production -D                                                     
    www-data 21786  0.1  8.3 148384 129260 ?       Sl   Feb26   1:48 unicorn_rails worker[4] -c production.conf.rb -E production -D                                                     
    www-data 21792  0.1  8.6 152156 133880 ?       Sl   Feb26   1:52 unicorn_rails worker[5] -c production.conf.rb -E production -D

Why would that be desirable? (I'm not a Ruby developer, so I have no idea)

I think Unicorn kills off old workers and spawns new regularly.