My point is that early on you have time to let the HoTs tick, and there's little point in topping people off any sooner as they'll be topped off anyway. Later on you need to worry about your long term (>10s) HPS just as much as your short term (<10s) HPS. Genesis lowers your long-term HPS by costing a GCD in order to buff short-term HPS. That's only a good deal if there's less than ten seconds left of the stacking phase. It can be a useful way to last one more Screech by dumping all remaining Rejuvenations onto their targets, but doing so will leave you with less healing for the next one and is only really advisable if there's no way you could heal through the next Screech.