Right, but without enough suction it's just not going to get any of the dirt down in the carpet, no matter how many times it goes over it. It'll just get light stuff that's right on the surface. Shitty upright vacs (so, the vast majority of upright vacs) sometimes try to make up for this with really aggressive brushes, but that barely helps and is really bad for the carpet—I don't think Roombas have enough room or power to even try that inferior work-around, let alone to generate a strong enough vacuum to really clean carpet.

That might be enough if the carpet starts relatively clean though - most of the deep down dirt gets pushed down through repeated use - stamping around a carpet with muddy boots will definitely ruin things quick but if you're conscientious about usage you're usually leaving surface level dirt that a mini-vacuum can scoop up if it's run frequently enough.

Roombas or similar things definitely can't keep a rug clean forever, but I think it's fair to say they'll make the times you need to get the heavy duty pet vac out less frequent.