I don't get the requirement for instant startup. How often do you reinstall your servers? It takes, what, a few minutes to sync for the first time a newly installed server. Are you trying to run your service on a Raspberry Pi or something?
I plan to run my service on heroku. The way heroku works (similar to Google App Engine I guess), is you prepare a server image (git snapshot), and heroku can bring servers up or down as needed. When a version is updated, new servers are started, then the old servers are downed.
In this setup, I do not keep a server that is necessarily long-lived ... I do not manage the lifetime of my servers. Therefore, I wish to make the bootstrapping phase as fast as possible, and spending 0 time on blockchain initialization is desirable.
Relying on a third party site like blockchain isn't just a trust issue. You are externalizing some of your costs onto Ben. Why should he pay for a part of your server costs, for free? He might do so for now, because your site will start small and he's a nice guy, but it's kind of rude to assume that if your website gets really big he'll just pony up for serving all your API traffic.
You could also just use the JSON-RPC API on a regular Satoshi node. It's really not such a big deal.
But the details really depend on what you mean by "various operations".
APIs are meant to be used. Perhaps in a future version of his API, Ben might charge for its usage. Then, I can have the choice of either paying to enjoy the simplicity that I like, or implementing/running my own server layer that handles this API. Even if I were to host everything on my end, I might partition my servers into two kinds - stateless web servers, and a blockchain server (running bitcoinj or bitcoind). I think it just makes the architecture cleaner, and lets me create this service faster without having to build the blockchain server right now, while still letting me create it later if I need it.