Yeah, it's about opportunity more than anything. If you just stick with whatever you get on the first try and wait for events to use all the lapis/tickets you get in the future you might get the best units anyway. Or you might never get Exdeath, CoD, and Cecil at all. Plus the very best units in the game are VASTLY more rare--even during their events--than the current best ones. If you want Lightning, you have to first get lucky and roll a rainbow (5*) crystal and then it has to choose her. Rerolling is just a one-time chance to start with exactly what you want, limited only by how much patience you have and the units currently available. If you roll up a good team now, then you can save all your future tickets for units like Lightning, Luneth, Chizuru, Gilgamesh, Majin Fina, Ace, etc. as they are added. Assuming you're even still playing then.
I'm rank 35, have a 5* ~250ATK Firion with all the best shit currently available, but that's about it. I've got Edgar who is probably the best 4* by a decent margin (ignoring the fact that he doesn't get a 5* once we get more JP content), but the rest of my 4*'s are mediocre (Kain, until 5*) or trash (Clyne and no-dual-wield Penelo). I can beat everything currently in the game easily, but if someone offered me an account with Exdeath, CoD, and one other 5* I'd start over without a second thought. I don't really want to start over though, so I'm making it a point to not try to roll some new accounts now.