NOTE: please read the dragon changes next season if you have not already.
These dragon changes will give far more substantial buffs to some champions over others (e.g. wind drake for Hecarim, Ocean drake for Rengar). It would just feel awful if I was in my ranked series and I lost game 5 because the enemy team happened to select a team comp better suited to exploit the 3rd drake buff of the game. If you give me the ability to draft around this in champ select, at least I will know I could have done something.
Also, it feels kind of strange to me that if the enemy team has a Hecarim, and wind drake spawns 3rd, that I will be more inclined to protect it from being taken as opposed to seeking it out for myself like usual. It's like I'm being denied the third drake and elder drake because I don't want to buff the enemy team.