What is the difference between using Drupal as a CMS and using it as a Portal?
Is this using Drupal as a CMS or a Portal?


I would say "CMS", and probably even "CMF" (Content Management Framework). E.g. using things such as Webservices.


  Does this go against the philosophy of what Drupal is for?


No, it's just 1 out of dozens of usages of Drupal.


  I want to use Drupal 8 if it matters.


Depends how much in a hurry you are, but I'd strongly recommend to not yet use D8 in production. Some more details on that:


Use Drupal 7 if you want to build production ready sites today, or in the near future.
Use Drupal 8 to get ready for "the future". To answer the (next) predictable question (= by when would I have to be ready for that), be aware of the famous quote from "Mr. Dries", which is something like: Drupal 8 will be ready when it's ready ....


Some more words/thoughts about D8


I do not think the D8 development architecture will change anymore (therefor it's too late).
So to start developing with D8 with a production date far away from today might make sense. And if you adapt to the new (OOP) stuff with Symphony, etc, then you probably don't need many more changes by the time D8 gets released.
Be careful however with an assumption like "... D8 is just about ready". Even if there are only a seven critical issues left before RC1.
D8's first beta was released at DrupalCon A'dam (2014). And I wouldn't be surprised if somewhere between Sept 17, 2015 and Sept 24, 2015 the first RC1 would be released (at DrupalCon Barcelona). But from then on an official release will not be just a matter of weeks (I "hope" for somewhere in 2016 ... and would "love" it to be in "2015" still).
The key challenge after D8 is released will be the contrib modules you'd need: if you can get the job done with hardly any contrib modules (now that Views is in core), then you're fine. But what if you'd depend on modules that only support D7? Or modules that don't have a stable D8 release yet, such as the Rules module?


Future-Proof your Drupal 7 site

To make transitioning to Drupal 8 easier, there are quite some decisions can you make now on your current or new Drupal 7 sites.

For more details on that, refer to the (amazing I think) presentation of David Reid about all those amazing contributed modules in Drupal 7, which are like backports of new Drupal 8 features.