Nobody is saying you can spend a lot of money and suddenly be able to beat a "very good player." Instead, you can spend money and gain a competitive advantage.

Actually my main problem with LoL is that you have to spend money to access champions. In DotA I'm used to having access to a huge complement of heroes, and a common strategy is choosing heroes specifically because they are powerful against the enemy team (counter-picking). In LoL it's a lot harder since you only have like 20 (or fewer?) heroes available.

Paying for champions is paying for additional content -- content that's free in DotA 2.