I have seen the feature discussed in multiple places (Bitcointalk, Reddit, Stack Exchange, Dev meetings, etc) over several years.
My understanding is that:
- There are some potentially negative privacy implications of using tx_extra because of possible fingerprinting from a rarely used feature.
- The use of tx_extra is still relatively uncommon after years of availability to a growing base of Monero users and developers.
- Most theoretical use cases such as refund transactions or messaging can be solved with other communication tools. In the case of Zcash use of the encrypted memo field with applications such as Zbay, appear to be result from a strong secondary motivation (to increase shielded transaction usage from a tiny base). Thankfully this is not as relevant to Monero where privacy is present by default. In fact the fingerprinting that results from tx_extra on balance appears harmful to network privacy.
I would love to here more convincing arguments about why the tx_extra field should be retained. Several of the better arguments I have seen discussed on GitHub relate to potential soft fork utilization and (theoretical) future development availability here and here. This padding idea appeals to me as a compromise (privacy improves while the feature remains) but must be balanced against the cost of blockchain bloat.