Ninja invites drive me nuts. I never ever accept, as a matter of principle - if you accept rude behavior, then you condone it, and it will just continue.
Is it a big deal? YES it is a HUGE deal. Why? Because its friggin rude that's why. Not only that, the person being invited has to do all the work. They have to see if the person that invited them is of a reasonable level. Once they join, which is always a big risk because of death debt, they then have to find out what the group is going to do, to see if they are even doing something the invitee wanted to do.
The work should be on the part of the group.
And no, its not ok to ninja invite if the person has LFG up. To me the same thing applies. You still have all the same problems - group compatibility (are their goals your goals), distance to group (in other words, death debt risks), level range, etc. Just because I am LFG doesn't make me fair game, if you ninja invite me I will still decline. I have no idea who you are or what you are going to do. Too many bad experiences accepting invites and ending up with a lot of debt and no progress.
Is it really that damn hard to ask?
Had one guy invite me over and over, 4 times, I declined without comment all 4 times. Finally he sends me a tell "we have a healer". That's all he said. So I says "for what?" and he tells me "we are going to get some xp". I says "have a nice time" and just ignored further tells. "we have a healer"? WTF is that supposed to tell me, is that useful somehow? I didn't have lfg up, I wasn't interested in a group, I was just passing through on the way to meet some friends. Instead of inviting me 4 times he could have actually sent a tell with some useful info. Its not like its hard to edit your tells and send the same tell to a bunch of different people.
Is it really that damn hard to ask?