Im cinvinced younneed variety for the exact reasons you say. This field is the best growing field we have hasnt been reseeded since the 50s when it was done with a little grey fergie and sungle furrow ploughs. Theres a lot of variety there i have no idea what most of it is but its dedinetly not ryegrass and clover. Parts of it look likena conservation mix we could buybfor herbal leys. Stock love it and will graze this in preference to the one year old ryegrass and clover ley the other side of the fence over the hill. Bare now ewes with tups need moving today.You wouldn't like a tractor if it only had two gears that worked!
They are a good start, of course because one fills in for the other - most of the time.
But what grows when they don't?
That's why folk have issues, and extra costs, to fill all these UhOh's...
I believe you need a variety of plants with a variety of different root depths and types to really cover your butt, and get full potential out of the soil.
Fortunately most of "the competition" think that's hogwash - which suits me fine!!
Seed is expensive but so is fert, and supplement, and another acre...