I'm not sure if it's ever been tested scientifically. Apparently the advice comes from Jonathan Ott though, in case that gives it any extra credibility.
In general though plant-based medicines have tons of different compounds, so it's important to change the strain every so often or else it A) stops being as effective and B) is more likely to cause side effects. E.g. if you keep smoking the same strain of weed then eventually it is more likely to cause anxiety then if you use a different strain each time. Essentially you want to prevent your receptors (and neural network) from adapting to what you're doing to them, and the best way to do that is to not do the same thing repeatedly.
N.b. this is not the same thing is polypharming, which is generally something to be avoided.
In general though plant-based medicines have tons of different compounds, so it's important to change the strain every so often or else it A) stops being as effective and B) is more likely to cause side effects. E.g. if you keep smoking the same strain of weed then eventually it is more likely to cause anxiety then if you use a different strain each time. Essentially you want to prevent your receptors (and neural network) from adapting to what you're doing to them, and the best way to do that is to not do the same thing repeatedly.
N.b. this is not the same thing is polypharming, which is generally something to be avoided.