Are Movement speed Quints really that good?

That's basically a free item passive. On the right champs and match ups, yes, its a HUGE difference. With how bursty these last few patches have been, there hasn't been many optimal opportunities to bust out the ms quints.

it could be okay against blitz maybe

All runes don't make a huge impact, but 4.5% MS is great. It's a percent increase, so your move speed will increase more sharply based on your items. Against a skillshot heavy opponent(Lux, Morgana, Ziggs, Nidalee, ect), it can save your life. Also, the extra speed to get everywhere adds up in importance over the course of the game. They are excellent quints, always useful even though sometimes others work a little better in certain cases.

they feel very nice to have so long as you're sure you won't get punished for the combat stats you won't have

I've survived a lot just from the movement speed alone. I honestly thought having movement speed wasn't that great or even worth having. I wish I had recorded this to show you, so I guess I can tell you it if you don't mind reading a small text wall.

Back in season 5, I decided to try movement speed quints out and was only able to buy 3.0% movement speed quints (2x). I went into my first game with speed quints and went as . I went to invade blue from and she ended up finding me. I tried running and she ended up slowing me. With bear stance I was able to have slightly and I mean SLIGHTLY higher speed (2-3ms more) and ran toward the enemy mid and hid in the bush with right next to me. I stunned her and ran back to the enemy blue, then tried to snare me, but could only slow me. I ran back toward mid after jumped into the bush in the other side of the wall. I ran back towards mid with only 100ish health and barely out running . Lux manage to hit level 6 and ulted, but missed. I ended up getting too close to and got slowed and I decided to just run straight back into my jungle and away from . I barely managed to escape that situation just because I had 3% movement speed from runes. So believe me that even though 4.5% doesn't seem a lot, it can really save you in a very bad situation like I was in.

Edit: Here is a video I saw from trick2g a few years. He ran 4.5% movement speed quints on top. They tried to gank him several times, but he was slightly faster than them. https://youtu.be/xOKKinEHHso?t=82

They're champion dependent and a bit better this patch.