I know 2 things very well. Cell site construction and combat operations. Communication is key to winning battles. If your player base is fragmented between multiple guilds without an underlying common chat channel your response time increase as does the likelyhood of losing that keep you worked so hard to take in the first place.

Aside from that what you describing is a Zerg mentality. I much prefer my tight knit guild that have all been playing together across multiple games. you won't have that cohesion with everyone time sharing between 5 guilds.

Outdated thinking considering ESO's five guild membership rule and because auction houses are guild controlled. You have to join a guild to buy things in their auction house. One guild might have a crafter who makes the weapon you need, another might have an enchanter who can make a certain buff for the weapon, yet another guild might have a crafter that can make the boots with the attribute you want, etc.

So there will be underlying common chat channel when I'm member of mega guild - regarding 1st.

Zerg mentality is ok if there is stability, I want access to AH and be part of group of 5 simillar guilds to cooperate, while I do not want to be member of one mega guild exclusively.

Anyone can purchase items from a guild store not just the members. http://www.youtube.com/watch?v=llYBrAi95d8 will answer the ins and outs of it.

Your missing the point. if you are a member of 5 guilds and you are currently in guild A (smaller guild) and guild C (Mega guild) needs you they have to hunt you down to request your assistance (unless all 5 guild chats are active at once and what a cluster [censored] that would be) With Guild allainces I can type one chat message and reach every member of the alliance without having to search for anyone. Clean and simple.

I have a feeling we wll have to agree to disagree on this one.

Giving players more freedom and options when it comes to guilds is not a bad idea, freedom and choices is always good. This also allows more casual guilds or rp guilds to thrive more easily as members may want to move on to larger scale pvp or pve while maintaining guild membership in their more casual guild, which will be possible.

It also greatly diminishes poaching members from other guilds, and can actually be used as a tool to build solid co-operative guilds.

"they have to hunt you down to request your assistance (unless all 5 guild chats are active at once and what a cluster [censored] that would be"

Yes, all 5 guild chats are active, but only the guilds I belong to (A - small and C - mega guild) I usually have also world chat open with 000's of players who can write there and I have no issue with that. Alliance of 9 guilds being one mega guild does not have to spam mega guild channel - why do you think it could be a problem?

"With Guild allainces I can type one chat message and reach every member of the alliance without having to search for anyone" - fine but even if all Allaince players listen, not all will be willing to run quick to help - they could even consider your messages as spam, they do not have AH to protect.

Later on when people join in PvP area there should be some coordination possible to discuss things - for sure not on world channel.

http://www.youtube.com/watch?v=llYBrAi95d8#t=746

But non-guild members would have to travel to the guild store in Cyrodiil.

The guild store is accessible by guild members at the bank in Alliance cities.