My thoughts on your points:

"1) It destroys communities: With the mix and match cross world system, it will destroy the union between players from their own world. It will be like 5k players playing their own solo games"
-----This cross server idea allows for casual players to have the ability to have their adventures when they want it. I can see why it rubs non-casual players the wrong way, but ruining the community, doubtful. It simply makes it possible for players to have adventures when their LS mates or friends are not available to go with them. I have heard from other posts that one can't build comradeship with cross server players, well that is because most of the time there is no system to keep in contact with that cross-server person whom you partied with. An option to add them to friend list, ability to chat cross-server, and the ability to know when they are flagged for cross-party seek can solve that. SE probably realized that server populations are so low that there aren't enough people in the same rank range, online at the same time, and want to party at the same time to make it happen. I for one want to party and fight monsters all the time, but there aren't enough in the server community who share that feeling and be online at the same time to form a party with. This cross-server system would allow me to have my adventure rather than standing around shouting for hours to form a party. I mean, who actually enjoys seeing tons of party recruit shouts for hours on end. So if server population is an issue, we can either consolidate server populations onto fewer servers, or make the game more appealing to casual players (increase populations in general). The latter makes more business sense by having more revenue potential, so I think that is why SE is considering cross-server partying. Cross-server partying does not destroy communities because those who care about their own server community will continue to party within the community. Are you are saying that by introducing this system, the players whom you regularly party with may betray you and use this cross-server seek system? You have nothing to worry about if the union between players is already strong in your community.

"2) gives oportunity for ninja's:
With less responsability for player reputations, you can and will come across people who see this as an invitation to take advantage of the situation and rob the party of everything they worked for. However, this does have a flip side and gives some players the chance to shout "ninja" every time they do not get their own way, regardless of what may occour."
-----This problem can be made less significant by disabling cross-server LFG for dungeons and raids that drop the best gear. If we use cross-server LFG for menial rank up parties it should be fine. It would be a good way for players that were left behind and outranked by their friends to make up the SP gap on their own time. I cannot expect my LS mates to always be online at the same time when our LS leve-link parties (where most of the rank ups happen) start.

"3) player conduct: Again, with less responsabilities for player reputations. Cross-world groups makes people think they have a right to pretty much treat everyone else like dirt. No amount of racist, homophobic, sexist or otherwise derogitory name calling (often having the F word seperate every other word) has gone unheard when this feature has been implimented in other games."
-----If by other games you include Wow, I can tell you from experience that it is not as bad as you make it out to be. Such prolific profanity was rare. A player would really have to be a *ick to get that kind of treatment. Such as AF-following for free exp, or trying to get the party killed. But that was usually resolved with a vote, and a swift kick, then you just have the system replace them. Now if SE implement an option that allows us to blist cross-server players, that may help. An example would be you are the leader and create a cross-server LFG group and the system would populate your roster with players who are not on anyone's blist. For each member already in the party, the system would check their blist and avoid recruiting those names.

"4) It has been tried, and has failed:
In other games, this feature is VERY unpopular, and has often made players leave said MMO's. I will not give the names, but i am pretty sure we each have our own examples of this. So, if it has failed and is unpopular in other games (often for reason above) why is it going to be implimented here?"
-----It was quite popular when first implemented in WoW, allows for players to continuously rank whatever jobs they want. It is a means to an end. A player can rank any job quickly if they put in the time. This allows them to catch up to y friends who have already reached the level cap. I rather have my buddies catch up to me so we can go do things together than have them looking from afar wishing they were there. Yes this cross-server LFG shortens the status quo gap by reducing the time it takes to catch up on ranks/levels. But on the positive side, it is a means for friends to stay within a level range even though there is a time schedule difference.