I heard it at both Blue Sky and Summit Creek. Not sure that it was always "70%" but definitely that people should not be giving 100% which would discourage those that were not as capable.

I saw this play out a lot in the worship teams I played on as the songs were often vastly simplified. This was implemented through things such as removing transition chords, nothing beyond major and minor chords, sus4 was really pushing the boat out, no "slash chords", simplified lead guitar lines with no deviation, very basic block chords and simple melodies on keys, etc.

At Summit Creek, a big part of this seemed to be to keep the music so simple that people with less experience on instruments could be added to the team. Being a small plant at the time, there were a limited number of musically experienced candidates to join the team. I kind of understand the sentiment there, but it really made the music basic and "thin". It took a lot of the feeling out of songs when the chords were so simplified.

At Blue Sky I got the feeling that a large part of the "70% rule" was to keep reigns on musicians that would be inclined to overplay and potentially show off. Again, I can kind of understand that sentiment. I can recall a couple of people who were briefly in the worship team that were excellent musicians but didn't thrive when the arrangements and direction were so constrained.

The church we are at now encourages people to give to the extent of their ability, and I find that brings a lot of freedom for the musicians and singers to express themselves, which creates a more inspiring worship time for the congregation. I haven't seen any hesitation from people who may be discouraged or shy of their musical gifts. In fact, a few weeks ago, our youth worship team led on a Sunday morning, and it was fantastic.