Saeviomagy said:
Reach isn't necessarily all it's cracked up to be.
Fully 1/3rd of the monsters in the MM have reach 10ft + naturally.
More of them have the capability to use a ranged or reach weapon, tumble, sneak etc, all of which negate reach.
Reach is HUGE. Having played several 10'-reach characters (a Glaive-wielding Psychic Warrior, a Spiked Chain guy, plus some monster PCs and an early version of the Half-Ogre), I've always found it to be a significant ability, the sort of thing that's worth an ECL boost in its own right. Sure, to get the most use out of it you need Combat Reflexes and some other utility Feat (Stand Still, Large 'n In Charge, etc.), but even alone it's a serious benefit.
If the enemy has 15'+ reach (NOT 10'), you're right, it doesn't make much difference.
If the enemy has 10' reach, it means I can step into the outer threatened ring, and hit them without taking a movement AoO. I've effectively negated their reach bonus (and as you mentioned, 10' reach is fairly common in the MM).
If the enemy has 5' reach, it's obviously an advantage to have 10'.
But it's better than that. If my Half-Ogre has 10' reach and I stand next to a spellcaster, he can't just take a free 5' step away to cast safely, because he'll still be in my threatened area. Same goes for archers, or people trying to use certain magic items. The increased threatened area also simply makes movement AoOs more likely; my Glaive-user was getting extra attacks every round, since there's always someone moving around during combat. They won't ALL be tumbling, and Spring Attack only protects from the defender.
Finally, there's the obstruction issue. If your campaign uses a grid map for combat, reach REALLY helps you get around the other fighters when they charge up to the enemy. When your group has a dozen characters (PCs, cohorts, animal companions/familiars/mounts), and the enemy has the same, things get crowded FAST.
Now sure, as you pointed out, the target could take Tumble (only applies to a few classes) or Spring Attack (third part of a Feat chain requiring above-average DEX). Neither of these are what I'd consider "common" solutions to the problem. Ranged weapons also aren't the answer; even ignoring the AoO issue mentioned above, the majority of characters use melee weapons. They won't just change this on the fly (or if they do, it'll be suboptimal). For the majority of opponents, the extra 5' of reach that an increased size gives you can make a substantial difference.
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Anyway, to the basic discussion, I'm one of the people who thinks that the SS Half-Ogre is overpowered at ECL+1. It should either be ECL+2, or (LA+1 with two Giant HD for ECL +3). To me, the two are equivalent.
But, it's irrelevant for me, since IMC we made our own reachless ECL+0 Half-Ogre well before SS came out. We switched, briefly, to the "official" version before deciding that whoever made it was stoned out of their mind. Ergo, we went back to our own version. (Short explanation: if a Dwarf can be thought of as "half Small", where it's a Medium with some Small characteristics, our Half-Ogre is "half Large". No reach, still Medium-sized, balanced stats, but needs bigger equipment and gets penalties to things like Hide.)
And to the original topic: having played Half-Ogres for a long time, I'd say to avoid letting players play straight Ogres. If all they can do is melee combat, but they're unstoppable at it, they won't be much fun to play and the game balance will get really wonky.