Togepi's 2nd evolution (Togetic) isn't really an "amazing must have" - it has some positive traits, but you won't feel any impact from it missing from your team. It can learn Fly though, which is nice. Togekiss (the 3rd evolution) would be the best to have on your team, since it is able to take big hits and also dish them out with high flinch potential. That being said, getting your Togetic to evolve to Togekiss requires a Shiny Stone, which requires a lot of hassle in HG/SS. If you already have a Pokemon that will be able to learn Fly, I wouldn't worry too much about Togepi for story progression. Personally, I didn't keep Togepi on my team until probably my fourth playthrough of the game.