The comma after serious in the second sentence is another joining comma. When the two joined clauses are short, as they are here, some writers would leave the comma out, as Dimcl suggests. I would, in part because the sentence will also be broken up by the commas around therefore. However, if you wish to be cautious, you should include it.
The commas around therefore are what the page calls "bracketing commas". They seem necessary.
I agree with Cagey. I like the use of the commas in the two sentences.
I guess I should point out that comma usage is something that has evolved over time (go read a Jane Austen novel if you want to know what I mean), and it is one of the most hotly debated subjects in English today.