Just my humble opinion as I have flown RF7.5 and RF8 prior to this using my field transmitter... beyond not having the wear and tear on your field transmitter, in my opinion, the new Interlink DX when coupled with RF9 is well worth the price. The Interlink DX when used with RF9 offers more channels than you can use from your field transmitter. I believe the max is 8, regardless of how many channels your actual transmitter has available.
The Interlink DX feels so close to my DX8 G2 with the switches and controls being in similar positions that it translates very well to the field.