This brings to mind one of the reasons that I think is responsible for summoning not being well covered in the first PHB.
Namely, I don't think summoning in 4E should be a simple thing of "use a spell slot to get a random weak creature to help you". This was fine enough in earlier versions of D&D, but in recent time, this no longer works as well.
The reason? Anime and videogames. Popular videogames like the Final Fantasy series, and popular Anime like Naruto have consistently portrayed a very different, and much more interesting, idea of summoning. In these games and anime, summoning is an extremely powerful and distinct ability, not just a normal subset of magic. Summoning always brings forth a unique (almost always intelligent) creature who has made a pact with the summoner, and is powerful enough to change the entire battle. Either the summoned creature uses an attack stronger than what a human mage can unleash, or it sticks around and is a powerful ally for a large part of the battle.
Overall, the anime/videogame approach to summoning is a lot more popular and interesting. 4E should do summoning that way, but doing it right will take a bit of energy and space that wouldn't fit in the PHB1, so I am glad summoning is not appearing right away.