A common problem at off-seasons is that one team will have to drop out at the last moment when it is too late to pull someone from the wait list, there is no more wait list, or there just isn't enough registered teams to make a full 24 or 16 robot elimination round. Having an odd number of teams also effects the way matches are evenly played. Most games are hard to win when you are short a robot in your match so it is a problem when you don't have a proper number of teams.
For instance, at Battlecry team 190 has sometimes provided a basic kitbot named "The Placebo" to used in matches when six robots are not present on the field as long as the alliance who is missing their partner provides a second drive team. I remember in 2009 the robot was used a lot and when one teams cRIO died with no replacement to be found they used the Placebo for the rest of the event. In 2008, 1519 brought their Speed Racer to Battlecry which competed on Friday for teams who did not show up. Later in 2010 BC was short on teams so 1519 used their practice robot dubbed 1915 for the weekend and team 40's practice robot was used as The Placebo (also used a lot that weekend).
I have seen several veteran teams use their practice robots to train new students/give existing students a chance to be on the drive team as well as offer to pre-rookie teams to compete and both are extremely commendable.
The only time I would have a problem with a team using two robots at an event is if it meant other teams couldn't compete. Practice/secondary robots should be entered into an event last or after registration closes so to speak.
I would also recommend that if a team has a practice robot, bring it to the event and let the organizers know in case there is a team that doesn't show up. I remember River Rage 2010 when the hosting team pieced together a robot at the last minute to make up for one team not showing up.
EDIT: Jane, personally I don't believe teams will show up with multiple robots with the intent of dominating an event. While it is true if you have two extremely capable robots the goal of the program and off-season is preparing new students for 2013. I'd recommend letting secondary robots in last, and the event organizers can determine if they feel a team is acting unfair (though I feel that won't be the case). Having worked in elimination rounds with an odd number of teams, what is more unfair is going up against another alliance with only one other partner because only 23 teams registered.
As for coaching, this is even better with two teams because you can have students leaders step up as "coaches for the day" if you are running low on mentors/coaches or have other coaches step up to the role. Only the team with the two robots will know what they are capable both with student and coaching power.