You are exactly correct, RESPA frowns on anything that appears to be "colusion" RESPA requires our clients to understand that they can use whatever title company they want and are free to pick and choose whom they want for home insurance etc.
The biggest ADVANTAGE to using the same title company is convenience and speed. There may be a slight cost advantage (under $50-$1oo) to using the same title company too. The title market is so competitive that prices are usually identical.
The only time it has been a DISADVANTAGE to using different title companies is when there is a problem with clearing title. Because title needs to be cleared with both the buyer and sellers side, using two different agents in different part of town (or the state) can cause an extra days delay or two to get both sides on the same sheet of music.
I have one or two title people that I LIKE to work with, but I ALWAYS defer the choice to my clients first, and if they don't care I'll use someone from my posse to get it knocked out.
This was probably WAY more information than what you wanted to know but I wanted to share anyway.
Best,