Plenty of people noticed the 4 man limit in Tyranny, of course, this was amongst the other more.. glaring issues with the combat.
I don't really look at it as a "oh I have to leave them behind", but only as a worry that they may take even more from Tyranny (obviously in this case the "bad" or "simplified" things). Things I think most people would rather not be taken, with multiclassing it bridges the gap between class specific and classless, and instead of 4 or 6 we get 5. I already am wary about how well multiclassing will work, and I take this reduction as a necessary decision because of it, rather than a magical right choice of finding its way.
We all know the balancing definitely took a while to come, and in Tyranny it may never be there, being the ultimate mage archer and fighter there was is something we all know people have been trying to prevent for years.
It seems multiclassing necessitates that you reduce the party size down, and may intertwine with this, but whether it ends up being good or bad probably isn't something to be discussed considering the game hasn't even begun alpha yet. They made two decisions that while not directly taking the actual simplified systems in Tyranny, seem to be a bridge a gap between those and the "complexish" systems in Pillars. I think as with most cases, the players who like being given the most challenge will notice it the most being the most use to every mechanic and their own quirks on how to manage them.
An edit, I see quite a bit of "well this game only had x amount of characters and it was fine!!!!" comments. And I think it is very unfair to compare games with vastly different combat systems, and visions to one of this nature. You could of course make that argument for many different features "x game had a voiced protag and it turned out fine". Some things can go against the core of the game vision, even if those games share a genre.