[quote name='banpeikun']Which strategy guide did you get? Just for reference[/quote]
It's the Brady Games guide. I guess it does tell you about the side quest and secret stuff, but it's not very clear. Telling me "future letter exchanges are dependant on other letter exchanges with other players" let's me know I need to do wifi, but it's not terribly specific. Plus, I spent about two hours looking for a certain character and failing to find her because the guide didn't say that she doesn't appear until you've sent enough letters.
Granted, the guide has made the game a lot less frustrating for me, and therefore more fun, then it would have been without it. But all it would have taken was about 2 or 3 more sentences to make the wifi and side-quest unlocking more clear.
Edit to answer Roufuss's question: Yes, you miss out on some stuff without Mognet, mostly highly level things. Apparently you need to exchange wifi letters to access the Onion Knight class, get the Ultima Weapon, and get all the master job items.