I think the usefulness of companions are in the eye of the beholder and/or circumstances. You may simply like the idea of a partner, or hate when they give you away while trying to sneak. I myself am caught in between. Sometimes I use them and for a while I do without. But one thing I do love are trainer companions. You can train with them, then enter their inventory and take back all your gold. Brilliant.
Companions stand as a complete annoyance to me haha. Not that they are bad additions to the game, they just find their way into my chain lightnings waayyy too often. The Destruction tree is my playground, therefore my spells are meant to kill everyone in the area that isn't me. Since chain lightning and other spells don't differentiate between who is your ally/an innocent and an enemy, companions just get in my way.
The first time I took Lydia out was a disaster. It was also the only time I took her with me to battle. I entered the ruined fort, started taking down forsworn, and after the dust had cleared, Lydia was nowhere to be found. Not even a body on the floor. Now she is missing and I'm not sure if she glitched or died.
Either way, companions just aren't my thing.
I was always a lone warrior, I never took companions in Fallout (except Dogmeat, but I left him at home most of the time.) I always found them to be annoying and getting underfoot.
Having said that, my orc husband and wife are making a good team in Skyrim! I'm actually enjoying it as they are both heavily armoured ebony wielding tanks. They don't sneak, they just hack apart everything in their way. Her support with the bow is handy and of course having her carry dragonbones makes life so much easier.