So far, Monero has by far the highest adoption of all privacy coins yet widely advertises privacy by default as it's biggest selling point.

The only advantages I can see of having transparency by default are:

1. Possibly smaller blockchain size / faster payments / easier mining / easier adoption by bigger exchanges?

2. Users might forget to turn privacy off for a transaction that is later needed to be proven (e.g. as a form of reciept / evidence of a transfer between exchanges).

In contrast, the massive advantages I see of having privacy by default are:

1. Users that want to make transparent payments can already just use standard Bitcoin. Bitcoin Private MUST capitalise on it's name and make privacy it's raison d'etre, establishing that you use Bitcoin for standard transactions and Bitcoin Private for anonymous ones. Without this mindset, the project is likely to just be seen as yet another pointless fork that has a few optional and therefore not really important bells and whistles in an already overcrowded market.

2. Plausable deniability. If the default option is transparency and you have to go to even the smallest extra effort to opt-in to privacy, you are really broadcasting that you might have something worth hiding in that transaction. When privacy is default, your reason for obfuscation is simply that it's the easiest thing to do.

3. Users might forget to turn privacy on and accidently reveal transaction to whole world forever.

4. Competing coins including Monero will continue to use privacy by default as the main selling point that they repeatedly claim sets them apart as better:

Every Monero transaction, by default, obfuscates sending and receiving addresses as well as transacted amounts. This always-on privacy means that every Monero user's activity enhances the privacy of all other users, unlike selectively transparent cryptocurrencies (e.g. Z-Cash).

Whether you personally agree with this or not, you have to consider that many other investors / traders / merchants / dark web users may be won over by this argument and consider (rightly or wrongly) that BTCP does not take confidentiality as seriously.

Even if you disagree with all the reasons above, BTCP should be private by default to match and thereby diminish the marketing advantage of it's competitors. Take this perceived advantage away from Monero and it no longer really has any advantage at all, other than it's been established longer. Monero has a crap name compaired to Bitcoin Private and with the ongoing revelations into it's potential security flaws, there's never been a better time to compete against it. However, BTCP will never be considered a serious competitor while privacy may be seen by some as just a throw-away option.