Giovanni returning to his evil deeds gives more for plot development with Silver. If he decided to be good and study pokemon, then Silver would probably be just like YAY DAD LET'S STUDY POKEMANS AND BE STRONG.As for Giovanni's backstory... I must say I'm slightly disappointed, though it was being profiled to get there since FRLG. I liked Gio in 1st Gen, when he said he'd disband the Team and dedicate to research Pokémon. That was cool, and also made Team Rocket actually kind of an extremist but effective path to fulfill that approach (Getting all kinds of Pokémon for study).
Unfortunately, FRLG changed that to a lesser alternative, as was "disband to get stronger". This really left his former character in the dust. Then came HGSS, and with Giovanni appearing, I didn't really know what to expect. Now, while the event is good in and of itself, and the backstory is a very important expansion... yeah. I also preferred Giovanni firm in his decision of not going back to Team Rocket. Of course, given the change in character in FRLG, it was to be expected. He still seeks power, and his reformed Team would seem to be the perfect way to get even more of it. It's not bad, but I liked the Boss when he seemed to have some kind of morale deep within.
Oh well. At least we get confirmation that Silver is Gio's kid... as if we needed it, but yeah.
And did Giovanni ever have any morale at all? He quit his job, disbanded team rocket, abandons his son, and hides in a cave to "train", all because he was feeling butt hurt about being beaten by an 11 year old rookie trainer.
Admittedly it was different through the original R/B/Y/G/S/C, but eh. I would rather they give more depth on Silver's story than just have Giovanni go "Meh, I got beat. Time to go dissect some pokemon"