When asking your question you added the comment that you always understood excommunication to be a biblical practice but consider self-exclusion to be more of a "slap on the wrist." Christian discipline is so vital to the well-being of any church, so I hope this answer is helpful.

Excommunication: Act of love

The command to deal with manifest sinners (see Matthew 18:15-20) keeps the practice of church discipline firmly on the Christian's agenda. The desire to retrieve and forgive the sinner is at the heart of it all. The parables that precede and follow the instructions in Matthew 18—the lost sheep and the unforgiving servant—emphasize this truth.

So it is wrong to ignore godly discipline or make it a low priority that seldom gets done. It's not right to assume or hope someone else will do this and save us the effort. It remains our duty to identify and expose sin among fellow believers and to seek godly contrition and joyful reliance on Christ. We are to approach them with humble love and the desire to serve and win them back to God.

But what if after our admonition, no repentance results? In love we excommunicate, declaring the impenitent sinner outside the community of believers. Unrepentant people show themselves to be out of fellowship with the saints on earth and in heaven, so we declare them outside—excommunicated—in the hope that it moves them to repent.

Self-exclusion: Act of frustration

As normally used in our circles, self-exclusion is not the same as excommunication. It is a way of terminating congregational membership but usually not declaring someone impenitent or outside the Christian faith. Congregations use the term when they feel they cannot complete the "steps" of discipline outlined in Matthew 18 (using witnesses, telling the church, publicly declaring impenitence). So congregations announce that those they are trying to admonish have excluded themselves from the church. First John 2:19 is sometimes cited, and formal discipline efforts usually end.

In practice, self-exclusion usually says excommunication is not procedurally possible or advisable in this situation, and frustrated congregational leaders settle for a termination of church membership.

Self-exclusion: Time to testify

Is this right? Is this loving? These questions are frequently asked. Every congregation needs to take a closer look at what it is doing and why. The essential thing is that a congregation thoroughly exhausts its efforts to reclaim the impenitent. Your view that self-exclusion is a "slap on the wrist" is incorrect since no Christian discipline activity is to be seen as a punishment, mild or otherwise. But your suspicion that declaring self-exclusion may be taking a shortcut and failing to serve the best interests of sinners may be in place.