Why?
Suppose I'm a 7th level enchanter wizard about to enter the Steading of the Hill Giant Chief. A CR 7 giant has a +4 Will save. With a 20 INT plus Spell Focus and Greater Spell Focus (as 2 of my overall 5 feats) I can cast my 3 Charm Monster spells (each with a 1 week duration) with a DC of 21. That's an 80% chance of success. That doesn't look that risky to me.
This is true, but only pretty useful outside of combat. If he even feels threatened, actually, he gets a +5 bonus on his save, cutting your chance of success down by 25% (to a 55% chance by your estimation). And, of course, any act by you or your apparent allies that threaten it break the spell. And that 55% is with two feats invested.
So, if the giants are okay with talking, you're looking good at charming one. This might piss off the other ones, though. And if it does, now you're in combat (or they're threatened), where your odds of success have dropped significantly lower than what you've offered. And, of course, just because the charmed giant sees you as Friendly.
Yes, Charm would allow you to try to manipulate it ("You can try to give the subject orders, but you must win an opposed Charisma check to convince it to do anything it wouldn’t ordinarily do. (Retries are not allowed.) An affected creature never obeys suicidal or obviously harmful orders, but it might be convinced that something very dangerous is worth doing."). However, I'm not certain that you could convince it to attack its own allies. Even the pfsrd says this:
PFSRD said:
Charm person makes a humanoid "friendly" to you, as per the rules found in the Diplomacy skill, but it also allows you to issue orders to the target, making an opposed Charisma check to convince the target to do something that it would not normally do. How does that work?
The charm person spell (and charm monster by extension) makes the target your friend. It will treat you kindly (although maybe not your allies) and will generally help you as long as your interests align. This is mostly in the purview of the GM. If you ask the creature to do something that it would not normally do (in relation to your friendship), that is when the opposed Charisma check comes into play. For example, if you use charm person to befriend an orc, the orc might share his grog with you and talk with you about the upcoming raid on a nearby settlement. If you asked him to help you fight some skeletons, he might very well lend a hand. If you asked him to help you till a field, however, you might need to make that check to convince him to do it.
No mention of making it attack fellow orcs one way or another, but I think my interpretation is probably more valid.
But once I have a hll giant charmed for a week I don't need to summon any!
This is the part where we disagree on what you can force the hill giant to do. It's very different from a summon spell. Maybe you're confusing Charm with Dominate?
Can't a wizard have Perception nearly as good as a rouge in PF (I thought only 3 less, and a wizard might be likely to have a higher WIS)?
I'd think the Rogue would have higher Wisdom, based on my experiences of 3.5. But PF did a lot to close the gap with how they handled cross-class skills, yes. (Also, accidental "rouge" on your part

)
For scouting, invisiblity and fly are reasonably effective, aren't they? (And neither is self-only, is it?)
Depends on where you're scouting. I don't use dungeons, really, but lots of people do, and invisibility isn't quite as useful there. But yes, they're very good scouting spells.
Just want to add that I think the saves in 3.5 are off-balance, I don't like class-skills in any form (including cross-class skills), and I think that fly/invisibility need a nerf. They're all problems I looked at in my own RPG. However, I think that we're still dealing with niche areas (non-hostile hill giant against an enchanter specialist that are met alone so that their buddies don't attack, and you win an opposed Charisma check as a Wizard [no retries] to allow you to do something that I think is against the rules [fight its allies for you]). But that's just me.
Edited to translate from 3.5 to PF.