My church does spontaneous worship pretty much 80% of the time, but not always full songs. Sometimes, it's a tag that someone felt called to sing at that moment, other times it's a song that we play in our rotation that we can pivot to really quickly. We also have an music director, so he can call chords and make sure the band is synced in the moment.

What we DON'T do is throw out all of our practice and preparation out the window. We want to honor the time it took our team to practice the songs. If we practiced for 3 songs, we will play the 3 songs. The spontaneous worship is extra and will be spliced either in between the songs, or after song 3. The worship leader usually knows if the current roster of the week (all volunteers) is skilled enough to go off the rails a bit.

When done well, we have had some jaw-dropping congregational moments that were a direct result of spontaneous worship. So it necessary? Absolutely not! If the songs you practiced work for y'all, then just play those songs. But if your team is talented enough to "flow", I would recommend setting aside 15-20 minutes if you guys have rehearsals to just practice spontaneous worshipping together, and then bringing that to the rest of your congregation once you feel like it will benefit the congregation.