Well I guess there's no answer for this
Not really, if it is the same keywords you already have in the article - i would have no effect. You'll have much better result rearranging the title (putting keywords closer to the beginning), or doing standard thins like marking the text -> keywords in bold, or list title, you kno, stuff like that. It was done because (I mean not taking account keywords in meta) when people start manipulate search mechanism and add irrelevant keywords to meta.
so I agree with what you are saying [thanks], will not hurt to have the keywords so why not?
The thing I do with synonyms - because you could not put all of it in a short article and I hope having it in meta might help SE to rank such an article for those who the search on synonym I didn't use. Don't know if it work - didn't do any research on this matter. Anyway, id doesn't hurt and code validtors do not count it (not having KW) as a mistake.
And BTW, there is "thank you" button - on the top right on every post.
Next confusing issue is the max amount of keyword phrases to use on each page's meta info, I have seen articles on the net mention no more than 5 while some say unlimited would not make a difference, any opinions please?
It depends on the % rather then total number. Use the keyword tool, plenty on line, it will show the %. Then, it's a funny thing, a lot of people agree it depends on the area. Some SEO specialist who have sites to do experiments say more then 5-7% in most of the areas would lead to article being penalized and taken out of SERP. I have a friend who runs colourpuncture sites. I did some wok for her at some point. In her articles 10-12% is norm. In one shee had ones 15%. Everything is indexed like candy. I have now clue why. My thoughts was - her articles are really long, very unique and colourpuncture doesn't have any synonyms.