You'd better say that you don't know how to use him, which would be more correct. For one, he has the best defence of all characters.
Each character can be maxed out and be a beast in various ways. Of course, when you get everything from every characters, there are powerful combinations. The key is to put the right gems on characters depending on the situation, or have a good default set.
With what they call Spikes in english version of the game, Reyn is just a killing machine on most enemy groups, and he doesn't even need to deal damage on its own. He can even get back life when he is dealt damage, and when an aura is active.
Every character is useful in Xenoblade, and depending on your style, you'll prefer use some combinations more.
What actually counts strategically in Xenoblade, and what define how you play them is :
- The 3 characters in your party
- The order they have in the party
- The gems they have
- The affinity between characters
- The skills they have
- The armors they have
- The weapon the have
You think only of damage dealing with your weapon, so you can't ever figure out how great Reyn is. He's not just a tank like in MMORPG.
All these DPS, Tanks and other Healer are meaningless in Xenoblade. There are gems that allow you to heal while dealing damage, get back HP while putting out auras, get back HP with chain Attack and so on. You can make nearly every character into anything, but of course some will be better in some areas.
The same is true with Sharla. Actually, you just show that you don't know how to control the characters, like most people that think they mastered the combat system but are far from it. I never had any problem with Sharla controlled by the computer, but I actually played with every character to max out the relationships, changed the Arts to suit my playstyle, and know when to go encourage my characters, when to tell them to go near me and stop attacking or go all out, when to prioritize maximizing aggro, when to prioritize tension, ... If you master all of that, you can beat any ennemy in the game with any combination of characters (yes, even the 120 level beast). If you don't, you'll be stuck with a few combinations that work better than others, and won't be able to do anything with the others.
But it doesn't mean the other characters are useless.
Whilst true that Reyn is not 'useless', he is definitely the least useful character in the game. If we look at the other characters (there are spoilers here!):
Shulk- All-rounder (Monado, healing, bonus damage)
Sharla- Healer, support
Dunban- Dodge tank, toppler, aggro drawer
Melia- Magic user, support character, toppler (starlight kick)
Riki- Health tank, DPS user, party gauge builder, support healer
Fiora- Any of the above tanks depending on equipment, DPS dealer, attacker
That leaves us with Reyn. And what role does Reyn fill? Sure, he's a useful damage-dealer, but Dunban, Riki or 7 arguably do a better job at it. If you use Reyn, you NEED to use Sharla, because of very poor defence for a tank. His Arts are mainly attack-orientated, and aside from being able to topple (which other characters can do as well), he has no real use.
I agree that the equipment, gems, affinity links, Arts etc. are useful in order to produce different playstyles for the game characters, each character is designed to fill a specific role. You'd struggle to get through all the bosses without a tank, a DPS, a healer or an all-rounder, depending on your playstyles. Saying characters DON'T have a special role to fill is simply false. They do, and whilst it isn't as evident as in other games, fact is they still exsist. And Reyn, sadly, just didn't cut it for me, namely because of his lack of defence, his lack of speed and his attack-only orientation. Yes, Reyn's spike is nice, but any character can gain Spike by equipping them with the gen anyway...
Personal preference of course, but Reyn just lacks something, and doesn't suit my playstyle.