Just like every other job we have places where we are useful and places where we aren't. You will be desired in areas where mobs are tougher than merit level and any time limits aren't very restrictive. Don't expect much love in Assault / Nyzul, Salvage and such. Sky, Dynamis, some Limbus, ZNMs and so on are where you will likely be popular post 75.
Luckily level sync and campaign battle take care of buffer and merits, so its not as big a deal there once you hit 74 where invites would drop to 0.
If you want to get into everything, It would be a good idea to figure out what you want for a 2nd 75 job. Unless your LS is very supportive you will likely need it to get merits in a timely maner, assault and salvage gear and merc rank at a respectable pace. DRG and DRK are good choices to give a try due to gear crossover. Askar, ares's and homam is top notch stuff for all 3 jobs. You can also skill & pre merit Polearm and Gsword on PLD, helps take care of some of the gimpness level sync would provide otherwise.
Edited, Feb 19th 2009 11:11am by Yashnaheen
Yash Nahain - Keeper of the Moon Miqo'te, DoM / Goldsmith
Ghostface Spidercarpet - Highland Hyur, Gladiator / Armorer
Don't know which one to keep yet.....