Speaking for myself, I prefer a good pre mixed cab IR to single mic IRs. Mixing cabs takes a lot of time that I’d rather spend doing other things, and so far none of my mixes is any better than the premixed ones I’ve tried. But there are very few mixes in the stock cab set, so I bought a couple of cab sets to get some more. Now I can spend more time on the amps, which is what I want to do anyway.
Also, certain vendors sound different, and people have their preferences. Once you play around with the stock cabs you’ll start hearing the differences between the different vendors.
But no, you don’t need to buy any, the stock cabs are very diverse and are as good as anything on the market, many were cherry picked from third party vendor sets.