I think we’re cool now. Sometimes people mix bits and bytes, not realizing the units are roughly 10 times each other or how significant such units can be.
I have no problem with your post that I just quoted. And I upload 20 gigabytes a day and download more than that, so speed is critical and so are units.
No, no, I get bits and bytes. I guess I left the unit off the 10 gigs of data, but I thought everyone would assume 10 GB, not 10 Gb. (Which you did, so I don’t understand the initial comment of the numbers not looking right, but at least we all agree in the end.)
Well, you know what they say about “assume” – it makes an ass out of U & me. 
n/m.
A T1 has significantly lower latency than cable and is a point to point line.
If lower latency is beneficial for your apps then T1 probably makes sense.
I work for a telcomm. In July I did a week long training on Special Service - this includes T1s T3, OC3 etc. As has been stated it is reliability that is bought. The average contract for a point to point circuit was service is back in 3 hours, with severe penalties to us if the customer is down for greater than .005% due to our failure. So yes, from a tech perspective it is locate the issue, fix the hardware or software if it is on our side, or prove it is on the customer side.
On who uses T1s, financial institutions have been mentioned, also medical practices, and design firms, really anyone who prises availability.