They do mix, except for Baruch Bay, nobody knows why they play by different rules, they get to greatly outnumber every other EU server and stay not full (and with no link). For some reason mixing English, French and Germans servers is ok, but not Spanish. Hordes of spanish speaking players from North, Central and South America also play on this server.
The NA system is designed so that all have links, the EU system somehow has BB and 2 unlucky servers which get no link for 2 months. It's entirely possible they never eliminated T5 in EU because of BB. Needless to say all linkless servers get stuck in T5 for the duration of the linking period with 1 going back to T4 every 2 weeks.
So, in essence, in NA you tank to get a more enjoyable match the following week, in EU you tank so that you don't have to spend 2 months with no link (or 6-8 months sometimes).