IMO, NIN and SAM are your main focuses for subjobs for party/alliance play. I can't think of a time I've ever needed to use anything but those.
SAM, I'd agree, mostly want to look at post-70, and even then it's situational.
Offensively, sure, top notch. Defensively, it relies a bit on luck (Seigan Third Eye -could- last...but it often won't).
NIN sub is consistently good for shared (or unintended) tanking. It lacks in offensive boosts if not dual wielding, but that's the price you pay for that level of damage mitigation =P As sharing tanking in some way is common, so is /NIN.
Consider DNC sub for soloing and the like. It doesn't start to shine much until post-30 (after you get Curing Waltz from level 15 DNC), but it's really pretty effective for lower-end soloing in the long run.
NIN and DNC particularly are subjobs that may prove to be a pain to level. I enjoy both of them, took them both to 75...but yeah, NIN doesn't start getting fun until 37, and doesn't -really- start coming into its own until the 60s. DNC doesn't get good until 40+ - Reverse Flourish -makes- that job...but anyway, they're both great subjobs for a -lot- of jobs, and worth the pain in the long run.
Most of the guides out there were probably written a while ago. Low level play has changed considerably withing the past months. You will often find people soloing (or duo/trioing/etc) in the lower levels. XP rates possible doing that now are comparable/better than partying had been. Granted, there are ways to get a lot better XP/hr in a group now too, but soloing can now net "acceptable" rates of XP.
As a consideration, planning on XPing at least some of your subs that way wouldn't be a bad idea. Yeah, it's slower, but you get all the drops, no commitment to stay due to a party, and so on. Do FoV, target EP-DC mobs, and so on. Decent XP/hr, more chance for defensive skills to skillup, and more loot.
____________________________•• Isiolia - Mithra - Pandemo... Asura FU SE ••
RDM BLM BST BRD NIN WAR PLD DNC BLU