And, to add my 2-penorth to things.
IMO the TP Gallery system is a handy "extra" thing for people to have access to...
be it for a newcomer who hasn't already set up an account on Flickr for photo hosting and sharing, or for regulars who may want to keep their TP Gallery for specific technical or setup stuff that they may refer to, or, for use as the OP's initial premise... or maybe a little bit of all of it...
While I think it's a good thing for members to share photos - both on the sharing sections and via some form of gallery - be it our inbuilt one, or externally via Flickr, 500px or where ever - and for the very reasons put forward by the OP and David (amongst others) - It's not really feasible to compel people to have their shots online. I'd expect anyone who's a "working pro" to have a portfolio website of some sort, and we do encourage people to add a link to this in their profile, and allow linking in their post signatures. But for "the rest of us", it's down to our own personal preference...
I'll admit, when I get critique from someone, and I don't agree with it, then I'll have a little look at what they've posted before to get a "handle" on their particular field... serves a dual purpose of a) knowing what level to pitch my reply to them and b) allowing me time to think over what they have said, and decide if I think that they might actually be right-even if I don't necessarily like what they say... So, for me, I'd LOVE people to link to a gallery of their work SOMEWHERE - I just don't think that the TP Gallery is necessarily the best place to do it.
(as an aside, I also realised that, if someone clicked my "flickr" link, they would think that I was obsessed by Tour De France mock-up race-numbers, and snapshots of various cycling paraphenalia, as I'd linked direct to my "photostream" which has been recently used as something of a "bit bucket" and had all kinds of crap thrown at it. So, if nothing else, this thread has got me to link my signature to a specific Album in the flickrsphere, which holds work that's probably more representative of my overall output)