I think it is less having to do with the tabard you wear but more-so having access to all the names on the guilds roster. I was in a small but friendly guild in Quel'dorei which was great for getting to 70 because there were people to chat with while slogging through the grinds. When I got to 70, though, everything changed. As gear became important and raids were the location of the gear, the rag-tag group of 50-69's that were great allies in the beginning became a hinderance to my playing enjoyment. I would look on the guild roster and often only see two or three names. Now Im in a guild of over 120 70's (even then still small in some terms) and getting a 5-man together for Brewfest Boss or MgT Heroic daily etc is a snap- something I would have to rely on PUG's for and risk getting ninja'd.

Point is, keep the old guildies on the friends list and the new guild on the guild roster obviously. Its easier to have all your old friends "friended" than it is to list all beneficial teammates in the friends list- especially when they can be guildies.