I and several other north east England 'Effers' would have agreed with you until four, maybe five years ago. I used to shoot a long-throat 7mm-08 in my club shoots in 300-500 yard matches, 600 too depending on the weather forecast, although I might equally have taken a 284 to them given the nature of the range I mostly compete on and its frequently difficult winds. Likewise, 6BR, its variants, 6.5X47L were widely seen at those distances in this discipline. For 300, I usually took a marvellously good grouping 7-08 load, but with poorer external ballistics than my 500/600 version, the 150gn Lapua Scenar-L over Lovex S065 (Shooters World 'Long Rifle' in the US) in batched and prepped older Winchester brass in a mild-ish load/MV. Not any more, nor my fellow competitors with the sixes and other smaller cartridges, and even less so, the considerable number of 223s that would be seen. For 300, I'll still use the 7-08, but only with a much more ballistically capable loading.
The reasons? A general and considerable rise in shooting skills, also the quality (and cost) of the rifle specifications and build standards, likewise ammo components and the kit being used to load the ammunition. (AutoTricklers V-whatever, plus added gizmos, plus FX scales creating a combo that costs getting on for the pounds sterling equivalent of $1,000 make my head spin and certainly won't be purchased in this lifetime!!) When a pack of very good shooters turn up with very expensive rifles with top-line March scopes chambered for the WSMs and 7SAUM in 300 yard comps, even the 284 starts to feel under-gunned. Our F/Military division gets increasingly large entries in these matches and these 26-inch barrel custom tactical/PRS beasts with very hot small primer brass 6.5 Creedmoor loads pushing 147s to impressive velocities almost hold their own with the best of the F-Class rifles to 600 yards. In theory, a good 6BR, even more so Dasher will also hold its own at these distances, but on most days' conditions they won't when winning every last 'V' ('X' in US) becomes vital. This is a combination of often really hard winds, flags that all too often don't change aspect immediately to reflect wind changes, plus - very important - we shoot 'pairs', often 'trios' in the UK with shooters following partners in turn, so a new wind assessment is often needed for each and every shot. On 'bad days' we won't get any number of 'possibles' or even any at all even at 300 and V-counts only in the low teens; 600 sees few 'possibles' at any time, and lower V-counts, especially as we use a target with slightly smaller rings than the international ICFRA F-Class or equivalent US equivalent.
The logical answer to the tyro, or competitor with budget constraints is the 284 Win as an all-rounder. Precision is potentially superb, and the external ballistics are more than good enough until you get beyond 800 yards. Barrel life is reasonable to good especially if loads are kept down a tad and a cooler burning single-based powder is used (Viht N165 is very popular here in this role). In my view now, the sixes and similar are fine to get into the discipline and learn the ropes for a season or two, but having no chance of a place in the top six quickly palls for people who learn those 'ropes' quickly and have the ability to shoot top-three scores. (Other than on those rare days which produce exceptionally stable conditions of course.) After a (not very long) while it becomes irritating to be constantly reminded too as to how much you're learning about wind-reading as somehow compensating for always being a few points out of the game.
Obviously, local conditions apply such as prevailing weather conditions, likewise the number and skills of other competitors, but in the UK at any rate we're seeing standards in some clubs that match or better those of national league fixtures of only a few years ago. Our pairs shooting too, the need to plot / graph everything quickly and accurately, and the '45-second rule' create a very different environment from US 'string shooting'. (The 45 second rule is that the competitor must make the shot on his/her turn to shoot within 45 seconds of the target reappearing, or nowadays of an e-target being updated after your partner's shot is scored.)
