timppu: Ok, but in your experience, how much are those purchases needed to play the game? Can you still do quite well in the game without purchases, or does it become increasingly difficult to keep up without them? What benefits do the purchases bring?

To give two examples from other "free-to-play" games:

Candy Crush Saga: while you can indeed play the game without paying anything, the game is constructed so that it becomes more and more wasteful to try to keep playing. Success in later levels becomes more and more about luck, it is near impossible to clear a level without getting many of those candy powerups, sometimes several in the row. And quite often you might be just a few moves from the victory when you run out of moves, and you are pushed to buy more moves to clear the level.

Team Fortress 2: Playing it totally for free is only really useful in case you concentrate only on maybe two primary classes. If you want to swap between several classes, then the small inventory becomes a real issue as you have to constantly discard "unnecessary" items to make room for new ones.

Each class has several weapons and items that are very useful for certain situations, so for a given class, you want to have many of them available for different situations. Trying to cope only with stock items makes the game harder than it should be. And who would want to play an engie without a Rescue Ranger anyway? Being able to fix your sentry from a distance, or teleport your sentry for long distances, with a Rescue Ranger is super useful, and a real tide turner in many cases (e.g. in order to smuggle a level 3 sentry fast to the backs of the enemy; I recall how surprised one enemy team was in Turbine when they were dominating our team easily... and suddenly they had my level 3 sentry behind their backs, mowing them down :). "Huh? Where the heck did that come from?!?".

Also, the game makes sure it becomes annoying if you run out of inventory space because it interrupts you from the game (when you receive an item but have no room in inventory), bugging you about either discarding something in your inventory, or discard that item you just received. So you have to stop playing the level just to juggle with your inventory. Just for that, you start wishing for more inventory space.