In what world of business is taking the path of least resistance being lazy? Making informed decisions that will help minimize the foreceable issues in a project is quite "good" business sense. Selecting tools and services that are well tested and well maintained by the very same community that you are targeting falls into that category. I didn't mention RoR simply because I don't have any experience with it good or bad. I'm sure there are others that can make a valid case for its usage with Bitcoins.
Not to say that he couldn't, but WHY would Piuk would write his own bitcoin deamon in RoR, when the C++ and Java versions are so fully developed. Does that even make sense to you? Are you suggesting that the OP take a similar route with .NET?
Oh yes I do agree about not rewriting the whole bitcoin deamon in C# when making a website. When needing to use a bitcoin deamon, it make a lot of sense to use a C++ or Java version instead, or even to use the hosted blockchain.info one (which is what I am currently using). But when it comes to things like needing to generate some bitcoin address / private key pairs, there is nothing wrong with developing this in C# and than using it in an ASP.NET website.
no worries. i've never used coinbase, but it looks like a very well developed website. i'll take some time to properly evaluate its services and the technologies behind it.
EDIT: do you by any chance have a link to the mention of RoR?
I did find this
http://news.ycombinator.com/item?id=4839772&buffer_share=7ab7bBuiltwith.com is an awesome site for checking what frameworks etc a site uses.
http://builtwith.com/coinbase.com shows it was built with Ruby on Rails.
http://bitcoin.stackexchange.com/questions/5287/how-are-popular-bitcoin-wallets-implemented also backs it up. Also looks like strongcoin.com uses ruby on rails.