^Basically

In HGSS+PokeWalker you can obtain almost every Pokemon in the Pokedex. If they added the starters, what would be the point of buying the other two Gen IV games?

This point doesn't really sticks. Even considering only the current player base (The kids that are supposedly cycled out each generation), only the players that got HGSS as their first 4th Generation game would allow the company to generate revenue for this decision. Plus, the game encourages meeting other players with other games and trade with them, so it's never a requirement in any Generation of Pokémon since it came out to buy every game. Only people that want to have every game, that don't feel like looking for other people to trade with (Not needed with the advent of Wi-Fi. At least not in person. And even then, the GTS exists, despite its flaws, though I'll agree the need of seeing a Pokémon so you can ask for it is inconvenient), or that don't have other people to trade with would end up buying every game.

Not that I agree with having the Sinnoh Starters in HGSS, but nothing's stopping them. The name of the game isn't "buy every game to get every creature", it's "trade with other games to get every creature". Whether it's possible or not for the player to do so is beside the point.

~ This ties in with the other thread about the special areas to evolve Pokemon, its like a version exclusive thing

No. This is entirely different, and speaks of the availability and efficiency of the game to provide you with as much content as possible. Pokémon that don't exist in a given version are absolute. The entire evolutionary line is impossible to get anywhere. All you can do is see it used by in-game Trainers, or mentioned in the case of Legendaries.

In actuality, what the lack of evolution methods does is detract from the game's value as a main Version. If I wanted to get every Pokémon in an easier way, I would get Platinum (Or Diamond/Pearl if I ignored the fact they're horrible) instead of HeartGold/SoulSilver. Not only I have access to every evolution method, but it allows me to evolve those Pokémon at my leisure if I want to raise another one in the future. Any other Pokémon I can't get at all in Platinum? I'd do what I'd have to do with any other game: look for people to trade with.

In HGSS? You have to trade to someone with DPPt, have it evolved there, and get it sent back. The only trade you should have to do to get an evolution should precisely be for Trade Evolutions, not for evolutions of Pokémon you can already get in-game, but can't evolve because they didn't bother to add a method. Whenever you want to train one, instead of just trading, let alone going to the evolve location, you have to wait for it to get evolved, and then get it back. Needless constraints.

You'd think they knew better after messing up with FRLG. At least in those remakes they were consistent and completely removed the elements that made the impossible Evolutions be impossible. There was no clock, and there was no way to raise Beauty. HGSS had the Deoxys meteorites, and even made Daisy's massages raise Beauty, making Milotic possible without Poffin feeding. Why the hell not add special locations for these evolutions?