Steam has it's place, but not for Civ 5 in my opinion.
Steam is ideal for games such as L4D, CSS, TF2, etc. Not really for Civ which is more of a single player game. The main problems are that if the Steam servers are down, then you're not able to sync with your account. Hijacked accounts are another downside. Players can use common sense and account verification to make it virtually impossible for their accounts to get hijacked but the fact is there will be a percentage that don't do this and will lose their game(s) to a hijacker. "You're account security is your responsibility" is the policy which for Steam it has to be. It just sucks to see somebody lose their game(s) that they used their hard earned money to buy, even if it was through their own ignorance.