Need Clarification On Lines Vs. Dashes

Please see the attached aria from The Messiah with regard to ties and lines and dashes. Is it absolutely required to use a line when a note is tied, across a bar line or even in the same measure? And is it acceptable  to use slurs in place of the line when multiple pitches are sung on one syllable? Are there variations to these rules?

Thanks

Always use lines and dashes for the full length of any melisma. Anything contrary to this is outdated notation practice. So the example above is correct, though slurs would be beneficial too. 

http://icking-music-archive.org/lists/sottisier/notation.pdf

Blam!  All the answers to all the notation questions ever.  This is the most recent set of industry standards for the publishing companies as far as I know.  Standardize thyself!  

Older editions of choral writing don't always include slurs everywhere.  The text spacing, dashes and lines were supposed to be enough to indicate them, but the new standard is to always use slurs.  It just makes it so much easier to sight read accurately at speed.  Particularly when a lot of text is cramped into a single measure.  I always end up writing the slurs in if they aren't there.  Otherwise it drives me batty.  (:

Makes sense. Isn't modern language weird? To take such a beautiful thing as a slur ... and turn it into a slur? How did this happen? :(