I am a hardcore tournament player who has played in Rogue Trader Tournaments from Salem, Oregon, down to San Francisco, Sacramento, Los Angeles and San Diego, California. I have also played in Jacksonville, Orlando and Tampa, Florida. Right now I am playing and living in Phoenix AZ, but the tournament scene is rather dead for a city of this size, so I might need to join a league.
I try to attend as many large tournaments as I can. Although the GW Grand Tournaments get a bad rap, I enjoy them, and my goal is to win a GT one day. Also I attended Adepticon in Chicago every year and if you are a fan of 40k, this is a must because there is no other event quite like it.
Why I choose some of the units I do in my tournament lists
There are a lot of reasons why I choose the units I do in my army. I will quickly go over some of them.
#1. It is a good unit.
These are the best units in the game and most people take a lot of these units. The best armies are nothing but these units or are built around them. We all should all know what these units are already.
#2. Adding synergy to the army
Some units do not work well by themselves, but in combination they are better than the sum of their parts. An Avatar in a footslogging Eldar list is like this. In a Mech Eldar list the Avatar is a poor performer, but you put him in with a foot army and he shines. He makes the army fearless, he is an awesome counter-assault unit, and a very good fire magnet.
#3. They are the models that I own. You can only play with them if you got them. This is generally not a problem for me, but it does come up from time to time when I am unable to finish a unit in time for a tournament, or I don't have, or can't get what I want. As Donald Rumfield said "You go to war with they army you have, not the army you want".
Here are the units that people look at the army lists on the boards and don't understand why you would take them. The answer is sometimes they look good, but may not fight very well. You know what they say "It's better to look good than to fight good!" Remember that most tournaments are not just a table top game, but a beauty pageant. You are being judged on appearance, so it may be a good idea to take a possible hit on battle points to gain some appearance points. There was one tournament where I was debating what units to take, and I took the better looking ones. I did lose that tournament, but I ended up winning best appearance, so in the end it was the right call.
#5. I like the unit
Here is another head scratcher to people who are looking at a list and wondering why certain units are included. For an example, I love the Thousand Sons. For the most part they were my first 40K army and I really like their fluff (Heresy for a competitive gamer!). So when I play chaos, I will always have them in the army. Are they are very good unit? Heck no! They are bad because with 5th edition everyone gets 4+ cover now so that diminishes their AP3 bolters, and their 4+ inv save. Add to that that they are very, very expensive, and most of the 5th edition codex's can cripple your psychic powers means that they are a massive waste of points. On the other hand Plague Marines are the best troop choice in the codex and I own 0 of them, and I have no plans to buy them either.
#6. I do not want to win with what everyone one else wins with. I could take a Seer Council or go full Mech Eldar (I own 2 Falcons, 2 Fires Prisms and 4 Wave Serpents that I have never used) or play with Plague Marines. I could play the full Space Wolf army I own or Mech IG that I have most of the parts for, but I do not wish to. If you or I win with a leaf blower list, what does that prove? That Mech IG is good? We already know that. But if you are able to win with a list that others do not think is good, then that shows that you are a good player. In the UK GT finals last year there was a guy who came in 5th or 6th with Necrons. Now that was impressive and showed that he was a very good player, rather than the guys who finished ahead of him with Chaos and the other top builds. Winning is good, but it is even better if you can do it with style, and a bit of panache!
So there are some of the reasons why I take the units that I do, and the reason my some of my lists are not totally optimized. They are my armies and I get to play with the toys that I want to, and I enjoy playing with. At the end of the day it is a game, and one that we have to have fun playing.