Ignimbrites, providing unique windows into magma reservoirs prior to explosive volcanic eruptions, are of two main types: (1) crystal-rich dacites, and (2) dominantly crystal-poor rhyolites. Crystal-rich dacites are typically homogeneous, while crystal-poor ignimbrites can display strong gradients in composition and crystallinity. This presents a conundrum, as the more viscous, crystal-rich units should be less prone to stirring and mixing. As ignimbrites typically erupt following a reheating event induced by recharge from below, this dichotomy reflects the competition between two time scales: (1) a thermal reactivation time scale that measures the time necessary to make a locked crystal mush rheologically eruptible (<50% crystals), and (2) a homogenization time scale associated with convective stirring. Using a well-constrained thermo-mechanical model of a magma reservoir, we show that the reactivation time scale of locked mushes is much greater than the time necessary to homogenize reservoirs by convective stirring. Hence, crystal-rich units, which require a reactivation stage, are inevitably well stirred. In contrast, crystal-poor magmas are rheologically ready to be mobilized without reactivation and need not be thoroughly mixed prior to eruption. This model provides an integrated picture of upper crustal reservoirs and has major implications for the link between shallow plutonic and volcanic rocks.