We are constantly adding and removing servers based on our traffic, and with something like New Relic we need to renegotiate or change contract if we suddenly needs a lot of servers. Pay by metric, pay by request etc. is fairer in my eyes. If I have 10 servers connnected and send 1 request-per-minute I still pay for having 10 servers connected.
If they process 1 request from me it shouldn't matter whether or not I have 1 or 10 servers connected, I should only pay for what they process.
The reason I am not using NewRelic personally is that my infrastructure is 5 linodes, which I pay $20 for each server. And with NewRelic Pro I have to pay $149 per host. I have seen people doing tricks like only enabling NewRelic for certain hosts just to get around their pricing model.
I really like New Relic, and its very pleasant to use their service, they give good insight about your application, I just wish I could afford their service.