Dmitri makes a good point that learning to play pente without the openining restriction will leave you ill-prepared to compete in Pente WITH the opening restriction. The opening restriction is always used in serious competitions. But that is no reason to eliminate the small board without the restriction -- many people are NOT interested in serious competition and would rather play on the free board. It is good to have both available.
The size of the board is significant (besides being viewable on WebTV). A smaller board takes away some of the advantage of the first player -- white with the first move is expected to be on offense and the smaller board limits white's ability to produce a winning line. Often games do not extend beyond 13x13 but the limited size does become a factor in some games.