A couple of points about this. First off, how much playing as another character changes up a game is subjective in the first place; due to differing play styles in general, one person could find that playing as Character B radically alters the entire experience, while someone else might find that it changes their overall strategy in maybe three places, tops.
Second, the way multiple characters are handled in games can vary wildly, period, and a large number of these are instances like the Castlevania games where the secondary characters' playthroughs tend to amount to "replay the game, except with most of the gameplay features and story stripped out", maybe changing one or two bosses accordingly (if they even go that far; some "alternate character" modes in Castlevania games literally have a character fight *himself* at some point). I've never played those modes, nor do I count them towards having "completed" a game because they're just not that much fun to me.
There are *some* games where it's important to play as both characters. Though it's not a game raocow would ever LP, a good example is Kartia, an obscure PS1 strategy RPG. You have a choice of two characters, and not only do each of them have a completely different campaign, with different battles, allies, and plot, but the two questlines are interconnected, and if you want the whole plot, you *have* to play both. Most games, however, don't do this, and vary in only minor ways depending on who you pick.
And that leads to my main point: In terms of an LP, unless the other character has an *entirely* different route through the game, such replays tend to feel more like padding than anything, as the biggest difference is generally "they play somewhat differently" -- which doesn't always really show up when watching someone else play -- and we've already seen basically everything else the game had to offer anyway. The Zero replay of Megaman X4 is the sort of thing I'm talking about. It had one different boss fight, a few different cutscenes, and required maybe a handful of different strategies, but otherwise, it was just a nearly identical second run through a game we just saw raocow beat. The only "new" stuff was seeing how Zero would handle different obstacles/enemies, but that didn't ultimately amount to much, either. Yes, MMX4 was short enough that it didn't make things take *that* much longer, but with a bigger game... yeah, it could get to be a thing.
And, of course, at the end of the day, we *are* just the viewers, not the players, here. If raocow decides he enjoyed a game enough to do a full replay with another character, great. If he didn't enjoy the game enough for that, or just doesn't feel it's worth his time (or ours) to do that, then we shouldn't keep nagging him about it, either. The same is also true with overly difficult or tedious optional post-game stuff.