Absolutely nothing wrong with allies

Look, I know there are a lot of people who hate allies and are unhappy with them. But there really is nothing wrong with allies.

No, no. I'm not saying there's no problem. It certainly is a problem when the meta is dominated by cookie-cutter lists such as Imperial Knights-Imperial Guard-Blood Angels. Or Ynnari with Craftworld Aeldari and Drukhari. When everyone begins to bring the same list to exploit some mechanic, there is a problem.

But the solution is not to ban allies. That's just, forgive me, dumb. Rather than banning allies outright or penalizing people for bringing allies, you need to give people a reason to take mono factions. Perhaps allied detachments don't get Chapter Tactics or Regimental Doctrines. I dunno. Or perhaps you can only use the Warlord Trait, Relics and Stratagems for your primary Detachment (the one with your Warlord) as Karru on Bell of Lost Souls suggested. Or perhaps mono-factions get more Command Points (I doubt this will work because honestly, if you're taking allies to fill up gaps in your army and cover weaknesses, those extra Command Points aren't going to help you unless you get Stratagems that help you cover those weaknesses).

Another thing that people seem to forget when they accuse the allies system of cherry-picking and having no downsides (which is plain stupid - there are downsides), is that when you bring allies, you're not just bringing their strengths to cover your weaknesses. You're bringing your allies' weaknesses along with them. If I bring Guard allies for my Knights, suddenly my opponent's anti-infantry weapons have a juicy target. Those same anti-infantry weapons would be near-useless and inefficient against a mono-Knight list. Similarly, if I bring Space Marines or Terminators to back up my squishy Guardsmen, my opponent's plasma weapons would suddenly be a lot more efficient than if they are used to shoot at cheap Guardsmen. On the other spectrum, if I bring Guard allies for my elite Space Marines/Deathwatch army, suddenly those flamers have something to shoot at, and those Orks would probably rather charge my Guardsmen than my Space Marines. If I bring a single Knight for my Guard, he's going to attract a lot of anti-tank fire, and once he goes down, that's a huge chunk of my list gone because I don't have other Knights - I spent most of my points on a Guard Brigade (and maybe Blood Angels).

It's the same kind of dilemma I face when I try to build my Thousand Sons. Sure, they are weak against vehicles and tanks, but if I try to bring in Alpha Legion allies for Havocs for anti-tank, I suddenly have a lot less psychic potential in my army. I'll have to trade a few Sorcerers, and probably a Rhino for my slow Rubric Marines. That hurts a lot, believe me. It's not a simple equation of bringing allies = no more weaknesses. Since you have a set number of points, you need to remove something to add allies. If you're playing Thousand Sons for psychic mastery, you need to remove some of that psychic potential to make space for allies. If you're bringing Guard allies for your Custode lists, you're taking less durable dudes and jetbikes with invulnerable saves and high toughness while exposing a good portion of your army to mortars and other low AP high volume of shots anti-infantry firepower. People who decry allies seem to completely overlook this and pretend that all weaknesses disappear if you take allies. It's not that simple.

However, it's also not a good thing if tournaments are dominated by similar cookie-cutter lists that bring the same allied combination over and over again. Perhaps a tweak to the allies system, reining in the Command Points farming, restricting Stratagems to the Warlord's faction, etc. might go a long way in mitigating that. But banning allies is just ridiculous. With T'au, Orks and Necrons being exceptions, you wouldn't be happy if you're suddenly told that mono-factions are banned and you must play allies. Similarly, it's not fair to ban allies just because you want mono-factions. Variety, choice and options are a great thing, and should be encourageed. Instead of smashing allies with a ban, think of ways to make mono-factions just as viable so that everyone can play their favorite combinations or mono-factions or whatever. Right now, I prefer playing mono-Guard (if you notice, most of my lists are mono-Imperial Guard and I rarely ally my Guard army with anything like Space Marines, Adeptus Mechanicus or even Knights) and mono-Thousand Sons. Yet I would also like the option to play allies (Knights with an Adeptus Mechanicus or Astra Militarum ally) because it would be boring to play the same mono-list over and over and over again.