I highly recommend listening to Coast2Coast pinball podcast episode #171. Nate interviews Josh Sharpe, and Josh discusses multi-kills and multi-kill scoring in the episode in detail. Bottom line: multi-kill scoring can be enormous.
Essentially, each time you shoot a lit yellow insert shot (by qualifying a multi-kill), you earn a fraction of your TOTAL AGGREGATE MODE SCORING for ALL MODES PLAYED, for the ENTIRE GAME up to that point.... and it DOESN'T DECREASE OR RESET each time you collect it, nor reset when you drain your ball. It only increases as you continue to score more mode points.
You do have to requalify your multi-kill yellow insert shot on each new ball. But once it's qualified and lit, you will always have one lit for the remainder of the ball, I believe. When you shoot one, you collect your multi-kill value, and the yellow insert moves to a different major shot on the PF.
Enjoy!