Why can't I connect to a Minecraft Server when I'm using 1.7.2?

I recently installed a shader that uses 1.7.2 forge and I can't connect to a server i play on when i use 1.7.2 however when i use 1.7.4 i can connect fine. But I can't play without my shaders on anymore so i'm not going to 1.7.4 because forge is taking the piss to release on 1.7.4.

Servers update differently to the releases. So that means one server might still be using release 1.7.2 but others require 1.7.4.
You usually can't connect to servers when they are a using a different version than you are. I guess just wait for forge 1.7.4 to come out, or just play without your shaders in 1.7.2