I play Magic: the Gathering fairly regularly. However, I also hold a steady job, am on the board of a not-for-profit organization, and spend too much time in front of both my email and the TV. So, my deck suffers from a lack of play-testing at times.
Play-testing is very time consuming. A match (best 2 of 3 games) of Magic can easily take an hour. If there are 6 other "top-tier" decks in the format you are playing, and you play each of them 20 matches, that's 120 hours, more waking hours than are in most weeks.
It can also be very deceiving. 20 matches is about enough to actually figure out your odds of beating some deck, and having a feel for optimal plays. However, it is far less than the number of possible games between those decks. Besides the number of ways to have each deck ordered after the shuffle, you also have the number of decisions each player makes during a match.
Suppose you decide to change your deck. Most of you old play-testing is no longer valuable. You've changed not only the choices available to you at each step, but how you would play the choices that you already had.
So, play-testing sucks. However, it is irreplaceable. I makes you a better player because you internalize common choices. It gives you an intuition for what makes a good hand and what needs to be mulliganed.
This was going to be a longer post, but I got distracted by actual M:tG play. More on a similar topic later.