Should you power up gym defenders

On busy gym yeah but low turn over gym you better off putting low cp pokemon.

nope

Im Mystic in area, where we dominate other teams twice or more with numbers, so i just put junk and have my coin usually. Sometimes i have problem to be kicked.

I noticed, that mostly Instnict players invest into many defenders. I use my only Blissey to keep gym before ex raid.

If you're in a high-activity area, it's worth it to have powered up defenders. They give you more time to defend with GRBs and serve as deterrents to attackers not willing to invest the time in taking down full-motivation Blissey, Chansey, etc. I happen to live in a metro area so it ends up mattering for me. Players in less-contested areas probably don't have to bother investing in level 40 defenders.

Seeing a Blissey Chansey or Snolax in a gym has zero impact on my decision to take it. If I want it, it's going down no matter what is in there. Don't waste your stardust on defense.

I'd say it's not worth it anymore honestly...

As someone who played in the old gym meta and having 2 maxed Blisseys back then who I DID raise to Lvl 40 when I was 38, the dust involved just isn't worth it for anyone newer...(and over 1.5 mil dust spent on random Rhydon's too).

As someone else said, most good trainers if no one is looking can tear down a gym extremely easily (like < 10 minutes). A lot of trainers also put random stuff so unless it's to defend an EX raid, once they kick 1 mon out, that gym is going down incredibly fast...

Just not worth it IMO for the stardust involved.

It really depends on where you live.

Yes if you want to actually hold a gym
No if your area is boring or you are part of the dominant team and need your defenders knocked out easily

I've done it, hell I still do it, but it seems like a waste of dust.

NO!
I dont get it why so many said yes?

The only use of a defender is to defend a gym for a legendary/EXraid. More than 1-2 defenders (which u probably already have) are not needed.