PoTM is one of the more unique servers that designs its balance not around the end game, but more around the middle, imo. And that's because characters have a general life expectancy of say, 13 levels. So keep that in mind too, if you're trying to examine the general balance or metagame.
You're probably right-- the difference between a lvl 20 (15/5 Barbarian/rogue) wouldn't be all that different from a lvl 20 (15/5 Barbarian/Rogue) who didn't didn't have the 10-level spread rule applied. But with no 10-level rule, the second Barbarian could do, say, 18 Barb/2 Rogue. The ruling doesn't say that all classes have to be a minimum of five levels. Only prestige classes. So without a 10-level spread, certain combinations of classes will be much more powerful than others.
I'll list a few:
17 Ranger/2 Rogue/1 Fighter -- A high BAB HiPS fighter with a full tumble dump
18 Barbarian/2 Rogue -- A barbarian with excellent rages, evasion, and a tumble dump.
18 Wizard/2 Rogue - A timestopping wizard with tumble dump, evasion, and enough intelligence to also be as good a rogue as dedicated rogues.
Okay, so the 10-level spread is good for making sure that dual-class builds can't reap the full benefits of one class, and a majority of the benefits from a small sampling of another to become hyper versatile, and with less obvious weaknesses. It doesn't necessarily stop triple-class builds from splashing a class for a few levels only, as I said before-- but I also recognized that triple-class builds often have no way of aiming for the higher end progression bonuses of any of their three classes. They're usually builds that aim for getting lots of early utility out of many classes that usually blend well together. Or a low-to-mid level caster with reasonable combat ability.
So if the only part of the 10-level rule you're looking at, then, is whether or not you need to have the levels in range of each other ALL the time, or only by the time it's a completely finished build, there are a few things you might want to consider that I mentioned above as well-- specifically, regarding the general balance focus on mid-to-high range rather than end-range.
The difference between a current rule lvl 15 (12 bard/ 3 shadowdancer) compared to a "late blooming" lvl 15 (14 bard/1 shadowdancer) is much more significant. Those extra two shadowdancer levels don't do anything productive for the current ruling bard (12/3). The late bloomer has better saves, better bard song, 5th level spells. He is better in every way. In fact, the two builds will only be equal when each build is level 20.
16.) Rule: 13/3 : Non-Rule: 15/1
17.) Rule: 13/4 : Non-Rule: 15/2
18.) Rule: 14/4 : Non-Rule: 15/3
19.) Rule: 14/5 : Non-Rule: 15/4
20.) Rule: 15/5 : Non-Rule: 15/5
*Could also be equal at level 18 if the non-rule bard wants to save his last level of bard for level 20, for the best skills.
This shows that frankly, yes, there is in fact a difference between a character with the current ruling compared to a character who could take his classes in any order as long as they -ended- 10 levels apart. This difference is visible throughout much of the late game. You listed some examples of barbarian/rogues, or fighters-- Those are harder to see, as they're classes that don't have obvious progression leaps. This power difference would be visible in every spellcaster, however.