NETWORK UTILIZATION. How many times do I have to say it?
It takes 2x as much Wi-Fi resources to transfer 1mb of data to a 1x1 client compared to a 2x2 client.
Capacity in wifi, especially in hotspots - is limited. Diversity is one of the most effective means of improving capacity, but it requires the clients to also support it.
The amount of data you can send per interval is incredibly important, and MIMO allows that to essentially be multiplied. The only other option for improving throughput without requiring more bandwidth is higher degrees of modulation and lower-overhead ECC - but that requires significantly more power, ergo range is highly limited, and increases the power-per-bit rate.
To negotiate MCS 28 or MCS 29 (1gbps+ @ 80MHz) between my Asus AC68 and RMBP requires the two to be about a meter away tops, and thats on the countryside where even 2.4GHz networks can operate in 40MHz mode with no interference whatsoever.