We use variables primarily for things like product names and other short strings that occur in broadly shared content on the one hand, and have a different value in different contexts on the other hand.

Apart from short strings, we also have a topic link as a variable, to point to each product's specific instance of a certain topic.

Although everyone's free to make variables, most are centrally managed, exactly because they are typically cross-product items.

I think our authors don't have a clear line between use cases for variables vs. conditions, but it seems like conditions are more governed and thus a limited short list (such as IVD or RUO), vs. things that are variable at a more granular level (such as variations in parts or protocols within a product line).
I work on translations, and I am forever cautioning authors about using variables or conditions on small text fragments, where the best practice is to do entire sentences or stand-alone phrases where possible. (Product names are an exception, as we usually do not translate them.)
Also note that long-term, we probably should be moving towards conkeys instead.

Regarding your poll on "How do you use variables" ...

We use variables in several ways, including (mainly) for short strings, but also occasionally for long strings or images. Not used for mixed content usually.

I do want to mention, however that we also use keydefs and conrefs extensively and some teams use conkeyrefs. We have a special mechanism to help our teams with keydefs see the contents of the key in Collective Spaces, but would be better if collective spaces supported keys out of the box. Conkeyrefs are not supported at all, and it would also help us if support for them was added to Collective Spaces.