The drawback is that .applescript files are not directly loadable into applications requiring compiled AppleScripts. There is a performance hit when compiling a .applescript file on-demand. .applescript files are fragile in the face of application terminology changes.
I consider .applescript files only suitable for sharing scripts with others and SCM because plain text is a lowest common denominator and is unlikely to be damaged in transit. I work almost exclusively with .scpt or .scptd files locally.
For me, the only time I use a .applescript file locally is when I have to save my work and the script is not in a compilable state. In these situations, .applescript is the only file format that can be used.
The nature of AppleScript makes it impossible to do 100% correct syntax highlighting without having AppleScript's knowledge of a script and the application terminologies on which the script depends (multi-word identifiers make AS's syntax impossible to parse without a lot of state and metadata). The effort required to re-implement all of this for the purposes of pre-compiling syntax highlighting just isn't worth it - once opened, compilation with AS is fast and there is a dedicated key - ENTER - provided for this purpose.
We do not auto-compile .applescript files when opening them (a) to keep file opening as fast as possible, (b) to avoid launching applications the script depends on, and (c) to avoid user interactions resulting from property definitions (e.g.
property homeFolder : choose folder)
After 20+ years of dealing with AS, the current situation is where we have settled.
It depends. For my open source stuff I use .applescript files in combination with
osacompile shell scripts to generate .scpt files. Internally I use .scpt files because Xcode can be difficult and this saves effort.
I also prefer compiled scripts (.scpt or .scptd) because they are not broken when a host target application changes the terminology in its scripting interface. When decompiling a compiled script, all terminology is drawn from the most recent metadata.