Online portfolios are GREAT if they are kept current, are organized well, and they reflect one's professional work. However, far too many artists use them as a dumping bin for commission sketches, early work, and stuff that, frankly, has no business being there.
A portfolio needs to focus on featuring images that will get you the job.
That's one of the reasons I prefer a .pdf, complete with all the artist's contact info on the first page and their name as part of the file name. This gives the artist a chance to go through material and choose only the stuff they need me to see. If they choose poorly, that tells me how they think and is a signal they may not be ready or easy to work with.
The other reason I prefer receiving a .pdf is that it's easy to circulate to my managers, to clients if we take you on, and to keep on my computer for immediate reference.