ASP.NET compiles. The stuff on your server is compiled.
Could somebody make changes to your site without the Source? yes, as you mentioned, they could decompile it.
But this removes loads of information from the original source; code comments are gone, for example. Many variable names as well, which are helpful for establishing a cognitive flow. Making changes to a website by decompiling what is currently there is analogous to trying to rebuild a racecar after a car crash. You have all the parts, but so many of them are unrecognizable and useless, and when you put in your own you find problems.
Basically if you try to get a developer to make changes to your site based on the decompiled assemblies, it will cost probably a lot more.