simpler theme/view mechanism/function?
There wasn't enough space in the comment field, so I'll write a full answer :)

As far as I know, there is no way to do that using existing Drupal modules or functions. If you really want to do something alternative, look into guidelines/recipes for creating own functions for simple templating parsing. 


  don't you feel that it is sometimes overkill/tedious just to theme something very simple with hook_theme etc...?


With the same logic, you could argue that you don't need any hook_menu() implementations, and that all you could do is have a .php file in the root of the site from where you would query the database directly :)

It all boils down to conventions because they imply more efficient/simpler maintenance, security, self-explanatory code for other people working on the same project, etc.

However, if you are the only one who is responsible for the site, you're free to do whatever works.

P. S. Toying around and stripping Drupal/turning it into something else can be very good for learning how things work at the higher level. Few years ago I maintained an in-house stripped version of Drupal 5 and 6 for own purposes, with different folder structure, without many modules/features I did not need and some other tweaks. It was a great learning experience.