I said yes to Horry, but would say no to Kerr (and Harper and Grant)
I said yes to Horry because I viewed him as a vital cog on multiple champhionship teams, even though Kerr and Harper and Grant were the same, I say no to them because I don't view them as important and vital as Horry. I don't think every player who contributes to champioship teams deserves to be in, just like I don't think every player with good to great stats deserves to be in.
Horry (playoff statistics):
'93-'94 Rockets: 11.7 points, 6.1 rebounds, 43.4% shooting
'94-'95 Rockets: 13.1 points, 7.0 rebounds, 44.5% shooting
'99-'00 Lakers: 7.6 points, 5.3 rebounds, 40.7% shooting
'00-'01 Lakers: 5.9 points, 5.2 rebounds, 36.8% shooting
'01-'02 Lakers: 9.3 points, 8.1 rebounds, 44.9% shooting
'04-'05 Spurs: 9.3 points, 5.4 rebounds, 44.8% shooting
'06-'07 Spurs: 4.3 points, 3.9 rebounds, 41.7% shooting
Career playoff averages: 8.0 points, 5.6 rebounds, 42.6% shooting
In terms of being a "vital cog", maybe you can make that argument for Horry's Houston years, but the rest of his numbers are fairly pedestrian. He's hit some big shots, but so have Steve Kerr and Derek Fisher.
Horry has won seven championships because he has been fortunate / smart enough to play with three of the top ten big men in NBA history, Hakeem, Shaq, and Duncan.