QUOTE (Katwoman @ Jun 8 2008, 11:12 AM)

1. I was recently in a workshop where the instructor said tags have no purpose, advantage, function and are not worth bothering with. True or false?
Your instructor would appear to be of questionable function also. This is a ridiculous statement. I'm not even going to bother qualifying that; it's just willfully wrong, and even cursory research on your part will show that. Whether they have any value to you specifically for any specific project is something for you to evaluate case by case. Next.
QUOTE (Katwoman @ Jun 8 2008, 11:12 AM)

2. What is the difference between tags and keywords? What is the purpose of each, and what differences are there in terms of how they are entered?
The most important detail to keep in mind is that they're entirely different things, regardless of their outward similarity. Tags are well,
tags as in the organizational method like you see on del.icio.us and other bookmarking services. Keywords are just extra words attached to a given entry as meta-data, and those words are included in regular text searches, even though those words may not be displayed in the template output. (So, for example, if you write an entry on tigers, you can include "big cats" in the keywords, and it'll turn up for that even if you don't use that phrase in the entry body.) I'm not absolutely sure of this(checking), but I don't believe that tags are treated in this way.
More practically, the Keywords field never seemed to be very widely used, and most people did and still do just use it as an extra text field.
Regarding entry, see below about tags(which also offer auto-complete). Keywords is just a plain text field whose content is not tracked, as any given set of words only applies to its specific entry. There's no formatting suggestion, either. Input however you like.
QUOTE (Katwoman @ Jun 8 2008, 11:12 AM)

3. If tags are, in fact, advantageous, what's the best way to enter a multi-word tag, e.g. "job interview?"
I've been using a plus (+) sign with no spaces between the words: job+interview. Is that the best way, or is there a better way?
If this is an SEO question, then you should take it to an SEO forum where many chicken bones will be waved and you'll get no definitive answer anyway.
As far as MT's
functionality, first off you can specify what delimiter you want to use in your profile. Then:
- If you use commas, you can just have a regular space in multi-word tags and they will be detected, since the actual tags will be separated by commas. They can also be grouped with quotes, but it's pretty much redundant. (foo, bar bar, baz)
- If you opt for spaces as delimiter, you can group your multi-word tags with quotes. (foo "bar bar" baz)
- Given the above, I see no reason to bother with the plus signs, though there's nothing stopping you.