The biggest issue, with higher grit belts, if they're flexible belts (J-Flexes) especially, isn't, as is commonly vilified; the splice, but rather, the cupping of the belt against the platen, which is caused by the flexibility of the belt, and the contours of the tracking and drive wheels. This is less of a problem on Bader and Wuertz grinders due to the shape of those wheels on those grinders, although that comes at a price of wheel tracking in many cases with the Bader grinders, although the TW90 has really excellent tracking and belt lay.
What's typically happening, is the belt, doesn't ride flat across the platen, and you're not putting enough pressure on the blade when grinding to force it completely flat against it. A soft backer can help mitigate the appearance of this, but it will also put you further and further out of flat, the more you grind with it. Not a big deal if you're doing belt finishes, but when you go to hand sand for satin finish with a stiff backer, you'll quickly realize how you're not riding across the entire blade because it's slightly convex. Many compensate for this using a softer backed sanding stick, but ultimately this'll show up in the lack of crips lines, and more subtle things the eye will notice, even if it cant quantify.
I often buy stiff Y weight belts at 220 and 400 grit, because when grinding really thin highly tapered little slipjoint blades or pairing knives, it's difficult to put huge pressure on them when refining at high grit, and doing so can actually cause a reverse problem of flexing the pressure point into the blade, when it's really flexible.
For thicker blades however, just put more pressure when you're making passes. However if you get the plunge tops too high, even with adequate pressure, you can see some cut-out above this spot where the belt is trying to resume it's shape somewhat, especially with really high belt tension.
On the other hand, if you're just noticing wierd facets at higher grit, it's probably a combination of technique and belt wear. Keep in mind, these higher grit belts are there for "removing" metal at any significant rate or for long, 3-4 passes per side is about the max of a 400 grit belt with steel IMO, before it's dull. I try to shoot for 2 passes per side, if that doesn't get it refined to that grit, then I didn't get it right in the previous grit. You need to be 90%+ at your coursest grit, 98% by the time you're done with 120 grit IMO, then it's just taking the finish higher and working out minor spot details like plunge angle etc.
Personally, while I find belt wobble or splice bump annoying, it shouldn't affect your results once you're comfortable and confident grinding. In fairness, a really bad wobble at course grit can cause you some issues in the plunges, but I tend to stay out of there at my coursest grit anyway. I like to push them with an 80 or 120 X or Y weight ceramic, get them exactly where I need them, then stay out of them entirely until I refine them at higher grit. I do not do, or like the swooped plunge effect however, and I don't make very thick knives typically.
