It could be nice... sometimes, you end up "previewing" your code by displaying it in order to get it to look ok. It could be easier to jus see it while writing the script.
On the other hand, I also want to see code in the current format most of the time. I agree that if it's optional, it would be helpful.
As far as that syntax highlighting idea... I never did like that in C++, VB, various HTML editors, etc.
And code completion would get in my way. I don't have a problem if it's there, but I type faster if I don't have to use code completion.
This would be really nice to use like a "preview", sometimes we are making some alias or remote and there is an error with color, bold, etc and with this option its very easy to find.
PS: Maybe with an option to Turn ON/OFF this option too, who likes use, who doesnt turn off.