Plugins do -not- need to be loaded each time you log in.
However, some macros will automatically unload plugins that can potentially create issues when using them and the macro at the same time. So KA does unload some plugins when it is being used and likely the reason they need to be reloaded.
Also, as @kaen01 said, if you are running multiple instances of the game at the same time, you would need to load the plugin for each instance of EQ. To avoid that you can load only a single character into Everquest and load all the plugins you want. Then load the other accounts in. When you do it that way the plugins you loaded prior to loading the other toons should be active on the other accounts loaded after the fact.
IE: Load Character 1.
/plugin MQ2KissTemplate
/plugin MQ2Cast
/plugin MQ2Melee
Load Character 2, 3, 4, 5, 6
Now those 3 plugins would be loaded already on all the accounts.
However, Kiss11.mac will UNLOAD some plugins when run. So on Character 1 you /mac kiss11 PullerTank
Now KA11 will unload MQ2Cast, MQ2Melee plugins for Character 1, however it's still loaded for Characters 2, 3, 4, 5, and 6.
Which ever character you unload last will likely be the one who has their plugin list saved to the macroquest.ini file.
So if at this point you unload Characters 2 thru 6, but keep character 1 logged in till last. Then you log out Character 1, then when you load back into the game the plugins will -not- be loaded on any character.