A buddy of mine and I were talking tonight about the the game Settlers of Catan, and the fact that I had commented the other day that I consider a win based on heavy investment in Development Cards to be lucky as opposed to a viable strategy. It turns out that we had both recently been beat (same game) by another young man who, as I see it, got very lucky with Development Cards; to the tune of getting three Monopoly (combined with perfect opportunitiest to leverage them), one Road Building, one Victory point, and just enough Knights to take largest army.
Given the fact that you can indeed win a game by primarily focusing your resources on Development Cards, it seems necessary to concede the point that “you can win a game this way”. 
However, the odds of winning a game this way are against you, and thus, I do not see any viable strategy herein; though there are certainly points that it lends itself to good tactics. The only time I see it as a good overall tactic is once you have built all five villages and you are going for a Cities + Largest Army win. My reasons are statistical, so lets look at some numbers – the percent chance that one will get a specific development card, assuming a new game begins, is as follows:
Knight: 48%
Year of Plenty: 12%
Road Building: 12%
Monopoly: 12%
Victory Point: 16%
A Knight simply isn’t worth the three resources you just spent, unless you are going for the largest army, which I wouldn’t begin doing until I had completed my expansion. Why? Of the three resources spend, two of them could be used to build a city, which not only yields a Victory Point, but also provides increased resources/production which can be used to upgrade other villages for even more points and resources; thus we loose opportunity when we get the Soldier. Of course, getting the largest army will provide two Victory points, but that is a lot of resources poured into Soldiers and something I simply wouldn’t do until latter in the game when I have the extra resources to spend as such.
The Year of Plenty card isn’t worthwhile, three resources (two of which help build a city) for any two resources you want? Absolutely not worth it in general, as the astute trader can typically trade those valuable city building resources to other players at profit and get what they want. The one time that this card is of value is when there is a shortage of some resource that you very much need, then this card becomes tactically viable – however, you are left with a situation where you must not only be fortunate enough to draw this card, but the games economy is also demonstrating the lack of a resource you are desperate for.
Road Building is arguably a cost effective card, you the get the production of what would typically cost you four resources for only three resources. However, I continue to believe that this is of little strategic use, while it can be good tactically. That is, you need to be in a situation where you can’t get the correct combinations of wood and brick in order to build your road, then you have to be lucky enough to pull this card out. However, with a 12% chance of pulling this card, you are likely to spend 24 resources before you see it (3 per card, 8 cards at 12%).
Monopoly is ranked right up there with Year of Plenty; however, this card as the tactical possibility to be devastating if played at the correct time. For example, suppose you know that all of your opponents have 2+ sheep in their hands, and you happen to have the wool port!? Now you have not only robbed your opponents of their investment opportunities, but you have significantly enhanced your own. That being said, the odds of you both obtaining the card (as with Road Building, you are looking at investing 24 resources to see it) and having such a stellar opportunity to leverage the card, are extremely low. Tactical, but not strategic.
A Victory Point is certainly worth three resources; yet it only has a 16% chance to be drawn, which means that you are typically spending 18 Resources (on average) to obtain a single victory point. Clearly this is a worse choice than investing in roads, villages, or cities; the later of which also increase your productivity.
As stated earlier, there are a couple of cases where Purchasing Development Cards lends itself to good Tactics:
- You have Seven Cards, need to dump some, and haven’t the ability to trade them in for a specific resource you need in order to build.
- You have all your villages built and are looking to finish up the Largest Army.
- A lot of cards have already been bought (by the other players) and the vast majority of those cards where Knights (increasing your odds of not getting stuck with a Knight).
However, they are all situational. In general, a strategy that invests resources into production that will increase resource yield will statistically prove to be the best overarching strategy and win more games in the long run, while playing the odds and getting lucky with Development Cards can win a game, and sometimes win it in a spectacular fashion.
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