I think the answer is, in the rare circumstance that SIP / T.38 is working perfectly, you would not change. We have big customers doing tons of faxing and they desperately need this solution because they can't get SIP / T.38 to an acceptable failure rate.
If you have an ideal setup - modern fax machine, fiber internet, correctly configured ATA (which is, by far, the biggest problem we've had - people just won't read the guides thoroughly), your expected failure rate is about 8% on a T.38 ATA. For small offices who send/receive a fax once in a couple of months, this is a fine solution.
HTTPS ATAs only fail when the party on the other end of the fax fails; which makes them at least as reliable as traditional POTS lines. They are probably more reliable because we will retry the fax 9 times before failing it. So if you need near 100% reliability, you need to use the HTTPS ATAs.
The reason we are charging monthly for the ATAs is because that is how we are charged for them. We have to buy software for these things to work, and its expensive. Most of our competitors who offer HTTPS ATAs charge north of $15/mo.
