''[[TODO: describe (only NRE-wiki page of relevance appears to be [http://tcl.msofer.com:8080/wiki?name=atProcExit+and+try/catch/finally]); rename the command to something more tclish?]]''
Consequence of [NRE]. 

[Lars H]: A thought: how is this different from what can be had through the age-old technique of creating a dummy local variable and putting an unset [trace] on it?
Basic implementation:
 proc atProcExit {script} {
    upvar 1 atProcExitScripts arr
    set arr($script) {}
    trace add variable arr($script) unset "$script\n\#"
 }

[MS] 2008-08-26 First please note that this is mostly exploiting the infrastructure of [tailcall] for a different and possibly interestuig purpose, but is definitely the least thought-out of the new unsupported commands. It might not be more than a "tech show-off" for NRE.

Now to the actual question: some of the differences are:
   * no need to find a variable name that is guaranteed not to be used, even in procs that may create and manipulate variables with names coming from the user
   * the state of the interp when the script runs: with the var traces, the proc's internal structures are still occupying the Tcl stack, and some of the code is still using the C stack. These run when everything is really gone, just like [tailcall] would.
   * well defined execution sequence (LIFO), which your construct does not guarantee (it is possible to have this feature with variable traces by putting the scripts in a list and running them in order within a [[catch]]) 

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