Lets put aside pace of progression and shared achievements/realm firsts where hard core 25 man guilds will be using this to an advantage anyways which means all these 10man raiding guilds will still need to work extremely hard to get a realm first. You have to ask yourselves without looking at wowprogress can you name the 16th best guild in the world for ICC 25 man progress?
I bet 95% of the people don't even know and I can promise you these guys work extremely hard to get that rank as we all know. What I am trying to get at is how important is even having a 125 ranked guild besides having good recruitment options? To most raiders it's being able to form a capable team to clear all content.
Why 25 man raiding (Bigger Guilds is good)
1) Community - 10 man guilds can be a very lonely place at times and lets be honest how many people do you get to know on your realm these days since Looking for group system was introduced into the game? Not many as now you don't have to stress about getting know tanks, healers etc to make up your group.
2) As a 25 man raiding guild you have the option to downsize to 1 or 2 10man groups when there isn't enough players online, also it allows the more hardcore members to downsize the raid
and try progress once the 25 man raiding schedule for the week has been done with.
3) Guild levelling system - 25 man raiding guilds will level faster. Also those thinking having a big guild and running 3 10mans is the answer, been there done that, it's a organizational nightmare.
4) Rated BGs within a pve guild - Much easier to do when you have more members.
5) To stress the otherside of point 2, strict 10 man raiding guilds will need a lot more dedicated members and will need to worry about class balance. You lose 1 or 2 key players and it can have a negative affect on the team. In my experience 10man raiding guilds don't last aslong.
6) IT FEELS MORE EPIC! :)
There are other small positives I can think of but those are more personal but I thought I would shed some good light on the 25 man raiding subject and why it's worth continuing with.
Most importantly have fun in whatever you choose to do.