You are seriously underrating what these pitchers can bring when at the top of their games. In his 17 innings of MLB experience Merryweather has accumulated 0.9 FWAR. Extrapolate that to full 60 or so innings of work and you suddenly see a 4 win reliever. Does that sound "pretty good" to you? I'm guessing that's a hard no.
Remember what the reliever version of Pearson did in the playoffs to the Tampa Bay Rays? If he's fully healthy and effective he's a player that struck out 5 out of 6 batters against a good hitting Rays team. Don't you think that's an impact addition?
Mayza has been an absolute stud this season. He had a rough stretch of 5-1/3 innings from May 7-24 where he allowed 12 earned runs. But outside of that bad stretch he's been basically perfect. Outside the short stretch where he struggled he's given up a grand total of 3!!! earned runs in his other 30-2/3 innings. Does that not sound like another elite bullpen addition?
Soria admittedly won't have the same type of impact, but his presence removes the worst from the middle reliever stash on the club, removing the weakest link from the bullpen.
Having a bullpen of Romano, Pearson, Merryweather, Mayza, Dolis, Hand, Cimber, Richards and Soria has a very high possibility of elite production. With the starting rotation mostly pitching well there wouldn't be a ton of innings to cover, and each member of the bullpen would likely get ample opportunity for proper rest as well.