A WHM/BRD post 50 can work well in parties without a BRD. Of course you cannot ever be expected to be the main refresher, but Ballad can help MP Regen rate and with a RDM it will be noticable. While you say that it has long cast times, it is however, nulled in that it hits every single mage within the area of effect, and therefore the effects are Ballad x how many mages and the PLD. Also a Madrigal before every fight post 50 helps. Evasion really hikes to a new lvl and hence why most melee buy themseleves a Life Belt(which is ironically a lifesaver for them, and yet some melee need MORE accuracy). While it will never be as effective as a real Madrigal, it will still provide accuracy up which will indeed be invaluable. But this is of course good only AFTER 50 with Ballad for maximum effect, and of course in those RARE parties with BRDs, this subjob is completely useless. So WHM/BRD is a valid subjob post 50 IMO. That is probably what i am going for in my SPT.

SMN however i disagree on the fact before lvl 50 because simply you do not get Auto Refresh. While you only get an extra 30 MP from the job. That does not even compensate slightly for Elemental Seal from BLM or Fast Cast from RDM, and not to mention other utility spells. You also lose out on Conserve MP if you sub SMN before 40 and then it doesnt offer Auto Refresh. In the end its always the rate of MP Regen for everyone, and the usage of MP that is better than max MP.

That lag thing i heard lets you take out SJ, tho theres no reason in doing so other than a personal entertainment thing. Best to put it back on before exping.

EDIT: You also would like to follow the general trend to maximize your job's potential, there are reasons why there are so many of those becuase they benefit the 5 other people in the party who also pay the monthly fee, and there to gain max exp. The point of these subjobs is that they are viable and they benefit you and the party the most. Party before self. Be happy that you can choose from SMN RDM BLM and BRD as subjobs and considered to be of some use. PLDs can only sub WAR for their role.