[IKE]

Doing 3-4 events per year is key to improving your teams ability to make a quality robot.
Teams that only do 1 event will often have some sort of "issue" that keeps them from performing as well as they believe they could. Having a second event requires that you fix said issue. With a fixed bot, you can see how it really will perform relative to your design. In theory you will also get exposure to another 20-50 robots/teams depending on how far you travel and what event you go to. This also gives your team a chance to revise their scouting strategies and improve them. Continuous improvement is a key component to many companies cultures especially in their development cycles.
By the third event, your drivers have enough practice that you can actually discuss and do tactics. This is part of the reason the Championship is so awesome. Many teams have competed in 2 regionals and are just then hitting their peaks of performance and capability. "Powerhouse" teams often do 2 Regionals and the Championship. Doing this many events is part of the reason they are "powerhouses" year after year.

A fourth event really tests the durability of the machine. Without a major modification, it isn't likely to score that much better. It will however give you a good idea of what systems were on the edge. At the MARC, I personally helped fix 3 of the 6 robots in the finals. These bots were really starting to show the wear and tear of the season.

For a young team (not rookies), the off season is a great way to get up to speed with other teams. In cerain regions, it is extremely cost effective, and has many of the same benefits as doing a regional.

If your ambition is to become a contender during the regular season, take the off-seasons seriously and try to improve your robot and team (scouting, pit crew preparedness, presentation skills...) at every event.

If your ambition is just to have more fun with the program, then it is a tougher call. You can treat this just like an engineering problem. Before mapping out the realities (like you have started), figure out what the team really wants to get out of the program. Do they eventually want to be world champions? Are they interested in being contenders? What are the expectations of your sponsors? These (and many more questions) should help define the goals of your team. From there, you can then compare and contrast the two different strategies.