One of the things I never understand is why people use a canon to kill a mosquito when all you need is a repellent (GoodKnight works well enough). I guess, besides lack of knowledge, the answer lies in human psyche – ego, indifference and sheer oblivion seem to be the most relevant answers.
One of the clients I have worked with earlier had bought BE license and was looking for an ideal project to start using the product. It ended fitting BE in a design where a simple ADB adapter would have worked. Even though we (and other fellow designers) strongly suggested the client to not go ahead with it, the design was passed and implemented. Why, you may ask. I am guessing the answer lies in one of the above mentioned reasons. Another client I worked asked me use XML canon just because they paid for it (was part of some package). I had to explain them version conflicts and other problems, and had some long discussions with the stakeholders and finally was able to take their approval for dropping it.
I have seen people use BPM tools such as iProcess for straight-through processing (STP). Unless there is a good chance that you may need to add human intervention later I see no point in doing that (STP is obviously much more efficient and faster). People going for database tables when just global variables can be used is another good example of overkill. Here is one of my favorites and probably the most common one. I have seen people give presentations and talk about (and try to implement) SOA when they fail to understand some basic design principles (this is not an exaggeration). When a solution can be just a simple shell script why create an elaborate package of BW processes unless there is a really good reason.
The point I am trying to make here is – KISS (Keep it Short and Simple). You do not need big elaborate plans to solve problems. You may think that consultancy biggies such as Mckinsey must provide their clients with some great big ideas, which is why they charge so much. But the reality is that most of the time the solutions they provide are real simple. What makes Mckinsey Mckinsey is the thought process that goes behind that solution.