Like when constructing a building... how high/big do you want it to be? The higher the rating you're shooting for, the better you'll want your foundation to be.
Drilling tactics is the fastest way for a beginner to improve. But as you may be guessing, doing tactics exclusively will only take you so far, eventually you'll have to go back and work on that foundation :) Tactics certainly never hurt you, but imaging that's all you do for 5 years, you're going to pick up a lot of bad habits though actual game play without really knowing it.
So for the short term, yes it's the fastest way to improve. For the long term, you'd want your study to be more balanced. So go ahead and do tactical drills, no matter your goals, just don't do them exclusively for a long time if you're in it for the long haul.
A mate told me that tactics is the surest way of improving ones game quickly, is he right? I've been doing loads of tactics and found it very useful in some instances but am not sure if it's the best way of improving