The reason why you offer unlimited plans is you have studied your customer lifetime value. If you have scientifically conclude that a your CLV falls X, you can consider playing around offering "unlimited" plans base on CLV averages.
One thing you also have consider is that you can offer some form of limitation on "unlimited" such as, support is free for the first year, or upgrades to major version not included.
There's a lot to mix and match with unlimited, but do not cut it out of the possibilities in marketing.
We have to acknowledge that Google has been performing up and down the perfect margin (but not getting it perfectly right), but it is because, presumably, they are in the last part of the equation to make it all perfect.
The only thing is they can't simply solve it relying only to technology because search engines deal with people. People are emotional beings, search engine robots, are, not bring funny, just robots.
More info here: http://www.mapoflife.org/topics/topic_431_Evolution-of-birds...