So what would the likely effects of this be? There is at least a good chance that it could serve to reduce the amount of diversity in the game rather than increase it. Let's consider some possibilities:
You're running an LE Quad. Ordinarily, you have to choose your hitters beforehand. Some people want to kick SWarm's behind. They put in two Zakyas and two Duergars. Other players might prefer to be better against other LE Quads. 3 duergars and a chrall are better against LE quads than 4 non-breath weapon/non-death burst hitters. Firebelchers a problem? You put in an Efreet. There are quite a few LE quad configurations out there. Now, enter the reserves. 50 points is enough for you to change out one hitter and all of your fodder or to change out your commander. So what are you going to do? Since I would no longer need to choose between the various LE quads, I could switch between a chraal and a zakya rakshasa as the fourth member of my quad. With the fodder, I could switch between a timber wolf and a greenspawn and still have room for my three pointers to be kobold miners, goblin skirmishers, or warrior skeletons, as it proved advantageous.
The end result here is that, at least in LE quads, the actual variability in the kind of LE quad an opposing player faces would be decreased. A shuluth band would never come up against kobold miners. A chraal band would never come up against goblin skirmishers, etc. In the beater department, SWarm bands would be a lot more likely to face down a zakya rakshasa (or multiple rakshasas than they are now).
How about an oldie but goodie: dual hill giants. The 50 point variation doesn't allow for the hill giants to trade out for anything else, but there would be room for the remaining 5 activations to change completely every match. Still, that wouldn't enable nearly the kind of customization that the LE quad is capable of. Bye bye hill giants.
How about frenzied beserker bands? Fortunately, a frenzied beserker is just over the point limit so a player couldn't decide on the fly whether a third beserker or a commander is more advantageous. He could, however, trade out the inspiring marshal for a gnome trickster. That kind of flexibility would render the trickster much more difficult to counter--if you make sacrifices to pack blindsight, he just doesn't show up; if you don't have a counter for him, or he counters you, he shows up every time.
How about SWarm? The only obvious answer is: ummm. be able to swap a warforged scout in whenever you don't need the standarbearer? With the 50 points, you could also replace two sacred watchers, but there aren't really many ways to mitigate the obvious weaknesses. You could pack Pippy to help against Zakya rakshasas (she can't be turned and she would enable the sword archon to engage more freely), but that wouldn't be enough to even up that matchup. (Though I think it probably would help a LOT against a Rikka Snake Eyes band which is another one of SWarm's bad matchups). The long and the short of it is that a strongly themed band like SWarm probably can't mitigate its weaknesses sufficiently to be helped more than it is hurt by the reserves rule.
From these thought experiments, I see a couple themes emerging:
1. Quads and trifectas have the room for customization that titan--and especially dual titan--builds lack. This would be a significant disadvantage for the quad builds.
2. Bands that have a counter would be significantly hurt. SWarm, for instance, would be much more difficult to play with a reduced opportunity cost for countering it and no really obvious countermoves for itself.
3. Factions that have cheap but game-changing pieces--like the gnome trickster would gain a pretty dramatic advantage in that it would become nearly impossible to counter both variations of their band.
In the end, I think the first question to ask is "is this a good idea" before we ask "how would we do this?" Based on the reduction in the diversity of warbands that would actually be faced (since you become dramatically more likely to see the counter piece to your band turn up and can include counters to other bands in your own band without the usual opportunity cost), I don't think it is a good idea.
Really, which do you think more when you're playing: "man, I could have really p0wned his a$$ if I'd been able to put a zakya into my band for just this game" or "man. Another freaking gnome trickster?!? My ranged band is worthless." The first doesn't seem like nearly the problem for the game that the second is. Do you really want more of the second in return for less of the first.