Explain purpose of M/F barrels on 28 ga. gun

[Wilmurt]

Need some education. Primarily I'm a bird hunter, grouse to pheasant and all bird sizes in between. Also have a fair selection of sxs shotguns in 20ga, 16ga and 12ga, no 28 ga though. I'm not a collector and shoot what I own. For 30 years my go to gun has been a 20ga Parker Repro with 3 barrels- 20ga Q1/Q2, 20ga IC/Full (factory original) and 16ga Galazan barrels with Briley thin-wall chokes. My hunting area is mid-Atlantic to New England. For its lightness, now comsisering purchasing a 28ga Parker Repro for grouse/woodcock because I'm getting a little older/slower and so Max, my setter doesn't have to wait as long for me to catch up. Good, clean 28ga PR's are at an affordable price point now, $4800-$5500 for a 26" single Q1/Q2 or IC/M. Two barrel sets are selling $6000-$7000 with 26" Q1/Q2 and 28" M/F barrels. Now to my question, what gunning purpose is the 28" M/F used? I always felt 28ga loads were too light with no oomph to cleanly kill larger birds. In tight grouse/woodcock cover you need shorter more open barrels, not longer and tighter. Is it worth the extra $'s to spring for the PR 2 barrel set or just stick to getting a PR 26" gun with the more open barrels?