As a member of a guild that does both private AND public raids, you get different things from both.
Private raids give you a clean environment to work on improving as a group: I firmly believe all training should be done in a private untagged raid. That’s not an insult to people outside the guild or their personal skill. You can’t see how your group performs until you put in a vacuum. You will never know if you’re being carried by numbers over skill until you can get a definite count on what you have. You also can’t hold members individually responsible until you can see what specifically they’re doing. For many people in WvW, having fun, tough fights IS the endgame. You simply don’t get that when the playing field is uneven, so you try to limit yourself to get it.
Conversely, public raids are fun. You can be more laid back. You can get to know people on your server outside of your little bubble. You get more willing scouts and better wxp. You help your server score PPT, which can be another fun game.
Both styles have their advantages and disadvantages and people tend to prefer one or the other for different reasons. Those people can and should work together. Part of that, however, is to have reasonable expectations and effective communication. If you see a fighting guild on the map and your t3 Garri is being challenged, send them a nice whisper asking to help out.
But keep expectations in line: If they’re fighting another guild at the time, then you are intruding on their enjoyment and they may or may not come. If it’s a close match, talk to them afterwards and ask them to please be more mindful in the future. If it’s not a close match: Save your anger for a time when it actually matters. I’ve been in the position of doing training on a Weds night on a non-push week when we’re 50k ahead – When someone PMed me about saving a keep in a map I wasn’t on when I was in the middle of a fight, I can’t tell you how little of a flying kitten I gave. Furthermore, that training makes us better fighters so that when matches are close and it matters, you have people with the skill to fight back.