Test has been around far longer than any other compound, I can't remember the first generation, but 2nd gen was pellets that they would insert under the skin. Dbol is a modified version of test. It is true that quite a while ago, people would only run one compound at a time, (AI or SERM in the beginning were not known; AI wasn't invented, but SERMS were), this was more to manage sides than anything. You couldn't run long cycles, stacking or heavy doses because you had no way to manage them.
As for running single compounds today, I feel it's a great way to get to know a new compound. The exception to this, would be compounds that shut down test production quickly (deca, tren, etc). Orals are really great to run by themselves, as you rebound really quickly (usually half the time you spent on them, is enough to recover). I know a lot of guys like to run high tren, low test cycles, but I'm more old school. I prefer higher test, low tren or low deca. I usually lower the total amount of test I am willing to do when I run other compounds, when I get up to 1k test, it's when I run test alone. I usually stay around 750mg of test, this allows me to workout 6 days a week with no recovery issues. I try to base my doses around how long it takes me to recover from workouts, rather than what the internet says.
For instance, usually when stacking dbol and test, I run 12-25 mg of dbol, and 500 mg of test. This allows my worked out muscle group to recover with 1-2 days off.