It is my least favorite part of the game (really the only part of it that I dislike), but you're right. I don't know if it was quite what Richard Garfield had in mind, but at this point it's 100% an intentional part of the design philosophy.
As it stands, it's something you have to consider and something you have to realize affects your opponents just as much as you. You can mitigate mana issues by playing a deck with low casting cost cards or by playing more land than normal, etc. There are also cards to search out land or can be used in a similar manner to land.
Yes, there is some variance, but poker has a ton of variance and no one would say that it detracts from the game.
The problem with having to add lands to power up your deck for me is that it means I can't throw in the kitchen sink, as I'm wont to do if left to my own devices. A deck design must first of all work - and that's 50% getting the manabase down pat, which includes tough decisions (should I trim out a basic land for one more copy of OP sleeper rare I'm sure shreds the deck-to-beat to pieces? Hint: no). So you have to build a deck while respecting some constraints (minimum deck size, enough mana for your spells, sharp strategic focus etc). But that's like, all the fun in deckbuilding (which is all the fun in the game, for me :).
In fact, I know this will sound eminently silly but I believe that playing M:tG and in particular bulding decks has taught me important lessons that I took with me in my career as a programmer. The most important of which is: don't ever waste resources.
Perhaps a more esoteric lesson is this: you can still have infinite variety even without absolute freedom. Even with the burden of having to include land in your deck and risk mana screw/flood, you can still design any deck you want and have a lot of fun while at it.