2 reasons why your last statement doesn't apply equally to Rugby as Football:
1) Club football has a HUGE reaching fan base such as teams like Man United, Chelsea, Real Madrid, AC Milan and Liverpool being supported all over the world. The coverage of their leagues is MASSIVE so consequently games like FIFA have a lot of interest.
In Rugby however, the RWC has a much higher global audience than the worldwide support for the domestic leagues. Many more people worldwide would know about the RWC than people who know about the Heineken Cup or the ITM Cup. Look at America for instance - HUGE market for gaming but many people don't have a clue what Rugby is. From my experience over there; there is very little club rugby shown on TV either but the RWC is shown and known by the people who have heard of Rugby, so an officially licensed RWC game is likely to sell better in a world cup year than a club game called 'Aviva Premiership 2011' etc that people in regions such as this won't have a clue what it is.
2) Many people don't choose to buy the FIFA international competition games because it is just a money spinning ploy by EA. Bar the last international game they released (FIFA World Cup 2010 South Africa) that had very much improved gameplay over FIFA 10, these game released are literally just the previously FIFA game with international teams, different commentators and a few more game modes, released purely to cash in on the global recognition for the World Cup, European Championship etc, so there really isn't much point in buying them if you already have the FIFA game. But, the fact that they release a game for the international competitions at all should indicate your statement is wrong, as it shows releasing an separate game is profitable enough to be a viable and a continued venture.
Also regardless of both of those points; HB would know themselves whether releasing a game in a World Cup year is more profitable than a non-World Cup year as they have done it before, and also gameplay is going to be the real deal-breaker for either game that is being released, not the amount of teams that it includes. If HB nail the gameplay, and therefore gain respect in the global Rugby community then it will pave the way for more games to be released in a successful series with expanded teams/comps. Ideally everyone would want all the teams under the sun, but HB have decided for one reason or another they don't want to (but that isn't even confirmed yet!) and we should trust their judgement on this. I for one would prefer excellent gameplay than a rubbish game with lots of teams.