The whole Xindi storyline was rubbish, it was simply an attempt to keep people watching, and it didn't do Enterprise any favours. Personally, I thought the series was a lot better when they were exploring new planets, and boldly going where no Beagle had gone before.
I'd have to agree that it was basically an attempt to keep viewers, but it wasn't a bad story arc. I found it the most interesting part of Enterprise, honestly. Season 1, 2, and 4 were pretty unremarkable. Aside from the Borg episode, I can't really remember too much. I enjoyed the series and would've liked to have it on longer, but it felt very different than the other series'.
Season one and two the Temporal Cold War, really bad Star Trek based TV show, and put the whole series on an alternate time-line, in fact the whole series was effected by other Star Trek shows messing with the time-line. in ST:TOS they messed with the time-line several times, Bones Vaporized a Hobo in the 30's by accident, Kirk interfered with Gary Seven, Scotty Gave transparent Aluminum to the 90's. Voyager Caused the computer revolution, and TNG left Borg in Cocrans time.
with that said I did like the last two seasons and wish they did the fifth season instead of closing the show in the holodeck with fatty too chubby.
as for the Xindi, some of them would join the Federation when it was formed but I think it would take time for some of them to join, but then try and think what may have happened it time travelers didn't mess with them, and then you might find a much different history.
but then things different in ST:Ent because of time travel...