How would this change things?

[Amechra]

What if, instead of using caster level, you use the Magic Rating system out of Unearthed Arcana?

And then cap any bonii to "caster level" at you HD, unless it explicitly splits the cap (circle magic would get this designation, as would a few other stuff, such as racial bonuses to "caster level")? So it's more like a magical BAB?

[Echoes]

It essentially replaces Practiced Spellcaster for everybody. This ... doesn't do a whole lot. The problem with losing spellcasting levels isn't the CL hit, it's the not getting ever-increasing piles of awesome part. The fact that your CL is 20 when you have 3rd-level spells is nice, but it isn't really meaningful in any way.

[Bloody Initiate]

I always liked the magic rating system, but I liked it for the reason I liked fractional bonuses. It doesn't have a big impact on the game, it just explains how the game already worked to people who hadn't caught on yet.

A lot of people never look at the game and reverse engineer to figure out how it works, they just let it tell them when to do something and accept it. This makes them worse at house ruling and home brewing. Home brewing may not seem like it's important/useful, but it's what you do every time you "fix" something.

The magic rating system just shows you how they originally figured things and applies the basic principle of it to allow a more flexible, albeit slightly more complex, game.