MIUI is great. But like I said, the stock launchers aren't really suitable. The keyboard lacks multitouch. What HTC and Moto have done really do improve on the OS. If Android came out a bit more polished, then we wouldn't have to rely on all this customization.
Customization is great too, but I think a lot of android requires customization to run well. Take the Droid 1. People thought it was the best thing since sliced bread. It might have been true, but I think the hype was that it was just the first seriou Android phone to hit the US on VZW too. Finally a challenger to the iPhone. However, 550mhz was just too slow. Thank goodness for unlocked bootloader and custom ROMs that took it to a new level. The out of the box experience was just mediocre to me.
You pointed out MIUI and Swype, but the point remains that out of the box, Android is a bit unpolished. If the stock launcher is nice enough and the keyboard is great, then people could hav ea valid point about HTC and Moto adding bloatware. However, as it stands I see SenseUI as being beneficial while MotoBlur is a bit meh. The multitouch keyboard they made is good though.