TLDR – The glow squid gets too much hate.

On behalf of the Caves and Cliffs update coming out, and that the glow squid won a three-way vote between itself, the Iceologer (aka Chillager), and the Moobloom, I can kind of see why they complain about it because it would only "glow" to the extent that enderman's or spider's eyes do, but here's why I am fine with the glow squid.

1. One one hand, you have two (or three if you count the brown mooshroom) types of cow; the normal cow, the red mooshroom, and the brown mooshroom.; meanwhile, you have four types of illagers (if you don't count the ravager); Pillager, vindicator, evoker, and illusioner. On the other hand, there is only one type of squid and adding the glow squid would hijack the one and change it to two types of squid. The warden would accommodate for the hostile mob niche anyway.
2. Glow squid have that cyan hue to them, and I love the cyan color myself.
3. Apparently, they are going to add glowing item frames to Minecraft that you can make with glowing ink sacs.
4. While then again, it may not glow per se, it could add some underwater ambience to underground ponds, where the axolotls would be found as well. To be fair, I like the idea of axolotls as well in Minecraft as they are the first amphibian mob for vanilla Minecraft.
5. If you're in creative mode, it could make a cool mob for your aquariums and zoos.