Your frame may be easy to work on, but that is probably the exception and not the norm.

I have two bikes with internal routing in my house. My Argon is OK to work on. Not great. The RD cable exits the frame under the BB and runs externally to the RD. That makes things a lot easier. Also, the way the cables are routed through the front of the headtube creates a cleaner run through the frameset.

My wife's Amira, on the other hand, is a complete PIA to work on.

I've got 30m of cable liner that I use to to make it easier to replace cables - basically, I feed the liner over the cables before removing them, then use the liner to re-feed the cables back through the frame. Despite this, something inevitably goes wrong. And it usually results in me spending way to much time trying to fish something out of the frame. And I usually end up sounding like the dad from A Christmas Story. ("Oh Fudge" is not exactly what I utter.)

Not only that, the internal routing isn't always the most direct route for the cable, meaning shifting performance can be reduced. And the cables often bang against the frame, making a lot of annoying noise.

As frames get more complicated, running cables is even worse. I spoke with a Trek rep a couple weeks ago about the new Madone. Initial set up on that thing is pretty difficult and time consuming - for electronic or mechanical. But once the DI2 stuff is in, it's easy to deal with. But replacing cables, well, the guy gave me a look that basically said don't buy this if you want to run mechanical. (The brake cables are much easier to replace.)