Simple question. Any real disadvantage to using a stem extender to raise a quill handlebar stem?

Note: for me, the weight of the extender is NOT an issue as I don't "count grams".

If you already have a quill stem (threaded headset), you need a stem with a longer quill. Look at the Nitto Technomic, for instance.

More likely, you have a threadless headset. I know of no technical reason why you shouldn't use a stem extender in that case. They're not very attractive, but that's an subjective judgment. Neither are the super hi-rise chicken neck stems, either. Either way, you can get the bars higher.

Technomics are pretty, adjustment range reasonable adding a couple inches..
the steel tube raiser has the stem wedge Bottom out above the top of the headset,
where the tube diameter reduction happens
so It's a big height jump. no middle ground.

I have a quill stemm. I bought a new quill stem and it was too long - so I'm looking at a quill stem extender to use the old stem. Let's not make this more compicated than I've presented it.

Simple question - is there any disadvantage to using an extender?

Bob is right. I use one and it was perfect for one of my bikes BUT you have to accept about a minimum 3" rise. You can go higher than that. They're cheap so buy it and try it. You may have to get longer cables.