[trevyn]

Ignoring tax implications is a much, much bigger problem for your argument than ignoring brokerage fees.

[sokoloff]

Indeed. Even execution slippage is greater than brokerage fees for a reasonable sized round-trip trade at a discount broker.

[floatrock]

Sure, the real world inhabited by the pro's is complicated, but the point is when we're first learning the fundamentals, we assume an idealized frictionless plane.