Being grappled would only keep the ogre from moving. In the mean time, he can beat the crap out of the barbarian or rogue holding him.
QFT. Grappling is not magic - all it does it stop a foe from moving. While grappled you can attack, talk, cast spells, use items, and so on.
When grappled, you can drop prone. This might be a benefit if the grappler has lots of friends with ranged weapons. You can't, however, stand up again until the grapple ends.
I do wish that the MM provided a proficiency bonus for creatures however.
You can work it out (but it is a bit tedious). The number of Hit Dice determines it's level, you can then refer to the table in the PHB to get the proficiency bonus. You could also look at the attack bonus, though those numbers are sometimes a bit more complex.
E.g. Bugbear (DM Basic Rules v0.5 page 13).
Its Hit Points are 5d8+5 which means it is a level 5 medium creature with a CON bonus of +1. Proficiency Bonus will be +3.
Its Attack Bonus is +4. Subtracting the STR bonus of +2 gives a Proficiency Bonus of +2.
So, +2 or +3, which is correct? The one from Hit Dice is correct, as the Attack Bonus is modified by the creatures Challenge Rating.
Some monsters have Athletics proficiency (there are 5 in the DM's Basic Rules). If they don't, their grapple escape is the highest of STR and DEX mod.