I'm on the fence about this one, but after all she came for free and I just use her services when I'm in the middle of a quest with a full inventory and reaching a hideout would take too long
I use her then I do the usual Orsinium run, sometimes I have a few items to much so I sell some to her.Why? I find she's very useful: you can avoid a trip to the town, with a couple of loading screens included. Is this worth a 30% cut? Normally yes, unless you have a lot of free time, or you have to sell items for several K gold.