"Adjacent mesocyclones had to be <75 km from each other in order to ensure that the adjacent cells within the line were close enough potentially to impact each other through processes such as precipitation fallout, outflow boundaries, etc."
75km/45 miles or less? That's like opening a hanger door to park a bike. I exaggerate but the call to chase a tail end storm is most intelligent with closer spacing than that if we are talking discrete supercells. Selecting only storm lines within 45 degrees N-S shows the researcher is bounding relative to the compass and not relative to wind fields and frontal or boundary placement, even if 45 degrees facilitates event selection and covers the majority of lines.
Despite somewhat arbitrary criteria prior to statistical analysis, there was enough rigor in the paper that their claim is somewhat validated. In a coarse sense, the southern storm is not always best. One has to wonder though, who drops south all the time, for any line without reason? I'd say most do it when storms are being starved of inflow or destructively interfered with by neighbors or a forming MCS- and that is the real criteria.
"Thus, the main question is: Does this tactic actually work? Will storm chasers be more likely to observe a tornado by targeting the southern-end storm?"
Anecdotally, I can think of many instances when the tail end storm was the only storm able to get clear inflow or stay isolated long enough to organize. It's also a favorable position for not getting squeezed into evolving clusters, as others have mentioned. The decision is not made in a vacuum without considering several factors and variables relative to each other that paint a big picture. I don't know many chasers who always go south without looking at what is going on first. If storms are being starved, then you might move. Maybe others do it differently but I have never had any blanket rule.