I've noticed that there are very few 25-man raiding guilds that only raid for one or two days. There's probably a significant portion of the audience that would be happy in this type of guild, and it seems a little bit unusual that these guilds are so rare. I think that there are two reasons for the lack of 1 or 2-day raiding guilds: the nature of guild leaders; and the pressure to do more.
Guild leaders, by their very nature, tend to be a little more hardcore than normal. After all, starting and running a guild is a great deal of work and effort. As well, they tend to play a fair bit. People who are only online one or two evenings generally feel that they aren't able to devote enough time to the guild.
Because guild leaders play so much, they tend to shape the guild so that it matches their schedules. If they want to raid four days a week, that's what the guild works toward.
The other reason is that there is a constant pressure on a guild to do more. Every extra day you can add to the schedule makes you better off. An extra day of farming, or an extra day of boss attempts, sounds enormously appealing. A lot of times I've seen a guild start with small raid schedule, and then realize that, hey, if we add another raid day, we could farm Kara for badges, or have two days for boss attempts, and their raid schedule balloons.
Lately, I'm coming to the opinion that a non-hardcore raiding guild should ideally raid one less day than it is fully capable of. For example, if your guild absolutely cannot raid for five days, but you can for four, I think you should raid three days of the week. Having that extra slack can go a long ways towards combating fatigue and burnout.
I think the next major innovation in MMO game design will be an pressure or force that compels you to do less, and is one which is willing accepted by the player base. For example, you could always brute-force it, and say that you can only ever be saved to one raid instance at a time, but the player base would howl. I have no idea what this force could be, but I believe it will be key to the success of the MMO that dethrones WoW.