IMO a lot of 'needs' are really varied.
For instance- are you locking up individual placements and need something with automatic optimization but kludgy interfaces or steep fees? Are you buying direct or through DSP's or Networks?
Are you running only on a CPM? Where/how much are you buying and from whom where numbers and being able to bill off yours is critical, OR is at a small/moderate scale feature set which you may not have/be able to build in house more important?
In my opinion/experience there's no 'right' answer, and with changes to display as quickly in the last 6-12 months it's vitally more important to REALLY understand the inventory you're buying, exactly where/what it is, how it's 'flowing' (since everyone and their mother is now an exchange, white labeling/reselling someone elses exchange/DSP, publishing shit through a ton of channels, etc etc etc.
The biggest downside to this is the bull$hit, which again to this point will make or break you, OR allow someone with a amateurish setup to win when some exchange tells you that X creative or Y landing page or Z product/offer isn't allowed on A inventory when in reality if you go around them, the exchange they're re-selling AND the display network this is going through and to the publisher they're FINE with what you want to run.
See some of those ads and wonder "wonder how party A got ad X on THAT site!??!"
Relationships/deal/money/creativity.
Not necessarily just being able to bill off Zedo. However to others points about getting raped on billing off others numbers this is correct, you will, you just need to figure out the percentage b/c failing w/ technology could be 200-300% deltas where billing may only be 20-30%.
Just my opinion/food for thought