Let's start with the definition:
Support Stamp - a picture with a thing and possibly text that you support it

Why I don't use them? I will write HOW I SEE IT

1. It's favourising someone over someone:
I use Support X stamp and Y could feel bad cus I don't support them(showing it)

I try to support everyone even thse that don't have a stamp
IDC if you are famous or not

2. It's a bit useless for me
I don't want to make a mess with that, it's just a waste of place for me
That's why I don't use any, I just don't have a need

I am less materialistic, that's why
I don't expect/want that someone will give me something
I am just neutral towards this

3. That's that you don't use a SS doesn't mean that you doesn't support something
It can be connected with the 1st point

IDC if you created one or not, it's your choice tbh
I cannot force you to stop or to do it

I hope I could explain a thingy :)
Feel free to share your opinion about using Support Stamps