The issue is the thicker caliper due to the thicker rotor, and the spacer just widens the axle (WMS to WMS) to match the rear, more or less. For most users, wider is betta. You don't necessarily have to have all that thickness really. You do have to have an IFS rotor to go with the IFS caliper though. If your wheels' backspacing clears the caliper without the spacer, or an adapter a la Sky or FRORF, yer home free.

-Bill
'87 4Runner w/ '96 5VZ-FE, 'Red Chili II'
'97 Taco XtraCab 3RZ-FE, 'BlackBean'
TLCA # 13257, Rising Sun 4x4 Club Land Use Coordinator
"He who stops being better stops being good." -Oliver Cromwell