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A month ago I posted a personal UML style guide, without going into very much background as to why I had arrived at that particular subset and that particular usage. And so I've been asked to "explain the reasoning" behind point 5:

"I use only two kinds of relationship on a class diagram: «implements» and «uses». No containment or aggregation (even when describing C++ code), no bi-directional relationships, no roles or multiplicities, and no navigability."

Firstly, I should reiterate the goals I have when creating a UML model. I use UML under only two circumstances:

1. As a design tool, to sketch the main collaborators in a scenario we're about to implement. This happens very frequently during a project, and is a 3-minute process of sketching on a whiteboard or the back of an envelope. The goal is to develop an outline understanding of how to assign responsibilities among the objects involved in one aspect of a system. This model is almost always a collaboration diagram, and will be thrown away very soon after it was drawn.
2. To create a map of a system as an aid to future travellers. I prefer to let the code and the tests do the talking, and so these after-the-fact UML models are intended only as signposts and summaries. Again, I focus on the system's behaviour, preferring collaboration diagrams, object state diagrams and swim-lane activity diagrams. I do usually include one or two static models, but each will document only a small, tight-knit cluster of classes; I never attempt to use them to show the whole system.