I believe there are lots of previous threads that revolve around this specific question, but I'll throw out my 2 cents.
A source is something that should be:
1) Confirmable - in other words, it should confirm the data in the profile.
2) Official - links to news sites and wikis and such makes the profile interesting, but it doesn't substitute the need for official birth records, census records, marriage records, etc.
3) Accessible - Now while I'm not a fan of paid sites, I will acknowledge that some records can only be seen through those sites. So I'm not stating that you can't use a paid site source, but the link should point to the source and not the tree, which leads to...
4) Direct - the source link should point to the... well, source. Not an interim link, not a search page - the source - it's OK to also have the tree link, or other profile link (like with ancestry, familysearch and geni, for example), but you should back it up with a direct link to the source, wherever it might be
Now my preference is that the source should be available as much as possible to non-paid customers so that the masses can look at it as well. Free sites preferred over pay. Non-sign up sites preferred before those that require you sign up in order to use.
And to answer your question (which could have been lost in the above) - I would not consider an unsourced tree link to be properly sourced. And while I would grudingly believe that a properly sourced tree link is sourced, it is challenging to use the link if you have to pay to look at it.
A good deal of this is opinion based, so please feel free to fire away if you disagree.