I too have used vbscript over many years and have a significant library of reusable components. The lack of an include was frustrating and I solved the problem the same way you have. The most significant problem with the approach is tracing errors when they occur. Because the entire script source does not exist in a single file, errors that occur reference line numbers that cannot be easily translated to the actual source line. Other than that it works well and has saved me tons of time assembling solutions from my library of script source class modules.
To the other point, admittedly vbscript is not a full featured developer tool and suffers from significant limitations especially on the UI front…however…outside of automating operational admin tasks vbscript can walk all over PowerShell. I see vbscript has a dumbed down flavour of MS VBA/Visual Basic with one hand tied behind its back. The power comes from the ease of use as an interpreted tool and the ability to instantiate class library methods using “CreateObject”. All of these tools have their strengths/weaknesses and if you actually use them you will understand when to pick what for a particular task. In an MS environment, between Visual Studio, VBScript, Powershell and ASP/ASP.Net there is very little you can’t accomplish from admin automation, to full blown classic client and web apps.