Catallus, the requirements of a 10-man guild are much different than a 25-man guild. The problem with a 10-man raiding group is that you generally need at least one of each class, and there are some classes that require you to carry multiples (paladins, priests, druids, among others).
To break it down, if you had one of each class, you have 10 players; if you have multiples of even half those classes, you will have 15 players/characters. Since most 10-man encounters require very specific class setups (2 paladins, 2 priests, 1 mage, 1 hunter, 2 druids, 1 shaman, 1 warrior OR 2 paladins, 1 priest, 1 DK, 2 druids, 2 shaman, 1 rogue, 1 mage), you might even need up to 18 players/characters, depending on the abilities of your raiding team. To compare that to a 25-man raiding guild, that is like having 38 to 45 players.
So, yes, he wants six people to join his guild to sit on a bench. It is the only way to do it. I know some people in HM 10-man guilds; they have 17 active raiders, and all 17 are online during each raid night.