Thrust bearings are really nice to have but not absolutely necessary for all installations. If I had one Ii would use it but I wouldn't be popping big money for one. From looking at your owners manual it looks like it has two races of 100 bearings each. My guess is it will handle your mast and light weight antenna with ease. Now having said that...
I have a Tail Twister 2X and it's rated at 1200 pounds sitting on that is a 20 foot piece of 2" x .200 wall steel pipe, a M2 KT36xa 6 elements on 32 foot boom, 8 feet of fiberglass mast and a M2 1 element linear loaded 40 meter dipole. I am using a bearing but not a thrust bearing.
Don't get caught up in the quest for perfection or you'll never get on the air.
here's you manual if you don't all ready have it.http://pdf.textfiles.com/manuals/SCANNERS-A-E/Alliance-HD-73%20Rotor.pdf