Imho the main point of having weight is that it allows one to execute a plugin before any other. What you propose isn't "simpler".

Ie. let's say i have two 'official' plugins, one with default weight (like most, 0), and another one with a weight of 10, i write a plugin that have to be run between those 2, i just set its weight to 5 and i'm done.
If we have high/normal/low priorities, and my first 'official' is 'normal', the second is 'high', i can't "insert" my plugin as easily.

Anyway, most plugins will have 0 weight, only few 'official' ones may have higher or lower weights (negative is possible), so basically only user's plugins may have different values.