According to Noonan, the weighted average of the grist should be around 10% protein. For all malt beers, a weighted average over 12% may cause haze or runoff issues. For beers using large amounts of adjuncts, too little protein may not provide adequate head, body, and yeast nutrition.
So a beer with 100% Maris Otter at ~9% protein would be ideal for a single infusion.
A wheat beer with 50% wheat malt (15%) and 50% American Pale (11%) would have an average of 13% protein and would benefit from a protein rest.
An adjunct-heavy lager with 50% 6-row (13%) and 50% rice syrup (0%) would have an average of 6.5% and would need additional protein to make an acceptable beer.