Fair/efficient/right thing to do?
Note, however, that in doing so you might end up overlooking some really great designers who don't have the time or expertise to show everything they've done online.
Depending on your needs, it might end up being better to go with a designer who has a lot of experience working on non webby things (i.e. print). Not that there aren't a lot of amazing web designers, but I've noticed that there tends to be a lot of groupthink in web design. Certain things become trendy, and many web designers end up just following these trends. Perhaps one of these less web savvy desginers have a more diverse background that would help you come up with something that really stands out.
This is certainly a good way to filter applicants out, but, you may miss a good opportunity as well. I hired an ex-architect with no web design experience at all (and no online portfolio) because she was cheap and had a great attitude. It worked out great for everyone involved. Can't say I'd recommend that for everyone, but it's amazing what can be accomplished by someone with the right attitude in the right environment.
Linking a portfolio on your CV/resume should be standard practice for a designer.
As for efficiency, it depends on how you define it. You will probably find your candidate pool is now relatively limited and it may take you a bit longer to find the right designer but it will certainly filter out those that just aren't destined to be the right fit anyway.
There is only one person that knows what the absolute perfect candidate for the job looks like and thats you. If that picture of perfection involves having an accessible online portfolio then so be it.
Say that a single attached example of best work might qualify, but an online portfolio would be better. You might like also to suggest a free hosting service for them to use to showcase their work.
Be polite, be constructive, but be definite.
Just my $0.02
I'm just posting these to share my frustration with the applications I get. The majority of them are so incredibly lazy. Spell check. If you're a designer, have an online portfolio. Etc.
And I share your frustration - I get equivalent things constantly. I have several form letters that can be sent with minimal effort to maximal effect. Depends how much you care.
But I agree that any decent designer should have stuff they can show you, it's just that I can imagine ways for it not to be completely cut'n'dried.
As has been mentioned though, you may miss out on some real gems.
I'm working hard to change it of late, but almost everything that I am most proud of has been done for intranet-work. I've built a number of quite impressive applications, all of which are shrouded in secrecy behind a firewall -- happily (and ably) servicing the requests of the customer. This means, of course, that none of that work is 'showable'.
I often ask permission of my customers to keep screenshots (with details obscured), and except for my government customers, most of them are pretty amicable, but of them, many won't allow it to be online where their competitors can see them.
Does this make me a sub-par developer? I don't think that it does. It does imply other things, of course, in that I might have trouble building to internet scale, for example, but I'm happy to SHOW you my work, it just might have to be in a zip file, or behind a password-protected website.
Somehow, that just doesn't sound right. You have to put yourself in the shoes of someone hiring. Make an effort to show me your work.
Password protected is ok, by the way, just send me the password in your application.
To put it into perspective though, most of my clients are federal government agencies, and yeah, to them, even a 100x100 pixel screenshot is tantamount to treason.
I just want to be able to discard really bad designers (and there are a lot of them) with a 30-second glance at their work. Similar to the supersimple coding test to weed out the developers who can't actually code.