I heard a lot about playing a game as original designed and intended. The truth about software development in general is that the released product may not be how the designer originally intended it to be. Game designers usually have bigger ambition at the beginning, then has to make compromises due to budget and deadline constrain. The designers probably don't mind more armors and weapons in the game, better graphic details, better NPC AI, etc., but they are always limited by time and budget, and also limited by the computer power at that time.
Therefore I don't think modding a game breaks how it was intended to play, as long as the mods stick to the lore and does not drastically alter gameplay. The main problem with modding Bethesda games is that you may end up spending more time modding than actually playing the game.
The KOTOR series is a great example of a game not being what the devs originally intended it to be, in that case, modding it actually unlocks all the hidden/locked out content and brings the game closer to what the creators considered complete.
As far as spending more time modding a TES game than playing it........ya........that happens. I cant tell you how many times ive got morrowind absolutely perfectly modded just the way I want to, and im either so sick of dealing with it that I just play somthing else, or I dont want to play it too many times because it will eventually destabilize and ill have to redo everything