Digital downloads have eliminated the scarcity problem. But pre-orders still serve a purpose for the game company. Pre-orders are money in the bank with no worries that bad reviews will slow sales down. In order to keep the money train moving, game companies needed a new way to entice customers to pre-order games. So now we have downloadable content available the moment a game is released. If you pre-order, you get it for free, if you did not pre-order, you can buy the DLC for an extra $3-$5. That sounds like a pretty sweet deal for the game company. On one hand they get money, on the other hand, they get even more money. That sounds like a business model that isn't going to go away unless they find another way which makes them more money.
So for the customer they must look at a few different aspects in order to judge whether preordering a specific game will be worth it.
First, how much playability do you think you will get out of the game? Have you been anticipating this game for a long time? If it is a sequel, will the game be enough of an upgrade over the previous version to maintain your attention? Are there any other games or significant life events coming up which might interfere with your ability to play the game? Would bad reviews dissuade you from purchasing this game?
You should always look at your own personal factors before you look at the secondary questions. If you are still intent on getting the game after asking the above questions, lets continue to the secondary questions.
Second, what will preordering get you? Do you get a discounted price? What DLC are they offering with the game? Are they bundling the preorder with another game? Where can you find the best deal that works for you?
Don't be afraid to look around at different options. Right now, Tropico 5 is up for preorder at multiple sites. Steam is the most well known with their tiered rewards. But you can also get the price down to as low as $27 if you go with Green Man Gaming and use one of their voucher codes. You can also get a copy of Port Royale 3 or Patrician IV if you preorder from the Humble Bundle Store.
If preordering is not for you. Don't be afraid to wait for reviews to come in on the game. Or to wait for a Spring/Summer/Fall/Winter sale.
Hell, you could even wait until you see Tropico 5 come up on /r/gameswap. Lord knows I have enough extra games in my Steam library which I might be able to trade for Tropico 5.