You basically summarized the pros and cons in your own post.
If you want a more professional looking program, I would use a toolkit that fits your needs.
If you don't mind the appearance of your program when made with simpler libs, then use the simpler libs.
EDIT:
Also you might want to consider how the program will function. People expect certain things from the behavior or programs. For example pressing tab will move between controls, pageup/pagedown keys will scroll the window, etc, etc.
If Allegro doesn't have this expected behavior, your program might be very awkward to use and will come across as rinky-dink and low-grade.
thank you Disch; what I understand now is Qt is for real projects, while Allegro is for simple programs (if not used for games) as same as using console applications in online algorithm contests, right?