If you have the slots for it, I think it's worth it... unless you read the "Digidone" thread over in Whatever Works and got a lump in your throat. I didn't.
I'd love to add a card, but thanks to my Mac Pro having only three slots, I have two HD cards and a UAD card, and I'm maxed out. I haven't investigated PCIe expansion options though.
I work almost entirely at 88.2 and I almost never run into a problem at mix time. But I do tend to print time-based effects, which means the vast majority of my plug-ins at mixdown are compressors and equalizers.
DF