engys account for like .5 percent of all play
same with heavys
and pyros
spies
snipers see a little more, especially if the map is beneficial to it.
Snipers see a lot of high-level play. It's not uncommon for a TF2 clan to have a very good ex-CS player that can headshot like there's no tomorrow. Sniper requires a higher skill level entry than Scout so most clans will run Scout more often than Sniper in that slot.
Engineers are used only in AD maps because CP is not good for the current engineer and the only CTF map played has no metal where you need it. Turbine has no metal in the middle so you can't go Engie. That's all.
Heavies are rare in the US but not as rare in Europe. However I have yet to see a single team that doesn't bring one in Well.
Valve is going to add the GRU or something similar to the Heavy sooner or later, for example. Given that everyone plays him in Well because he is not 5 seconds behind everyone I'm sure it will open many doors. Yes, he won't be played in Badlands but he will be played.
Something very important, this WILL change. Valve is balancing the game day after day so the "character tiers" are not going to be the same. Make your map good for all the classes you can and then let the players choose not to play Pyro, not the other way around.
If you ignore this your map will probably get obsolote sooner or later. The metagame changes and the game is evolving.
Also competitive doesn't mean "using the same tactics". The game will be more competitive if more classes can be played a profesional level because it will require more strategy and skill.