I used to do war/whm to solo, i think i did it once in a pt in valkurm dunes a long time ago, lol. I remember i used to miss alot, hit for 0 damage with my axe then cure, rest after fights for mp, etc (this was when 20 war's were seeking and 1 whm an hour appeared). I've never thought about it since then.
Although, it is a different idea i suppose, as you said not the BEST sub to choose (you can use ninja for utsusemi). I guess it's a good enough sub, with the right PT setup(and the right race, galka and elvaan mp would be extremely gimp, at level 69 BST/WHM my MP is a low 107mp base).
BUT that being said, your defense gets gimped if you sub WHM when tanking, so... I can't really see how much benefit it would bring, imo subbing PLD is proberably better as you get the DEF gain as well as the cure, although.. your MP gets gimped even more.
Personally, i think NIN > WHM/PLD sub for a war in a PT at that level, who is tanking and doing whatever.. it's more 'versatile'. Utsusemi means you get hit less.. means less cures.. means less downtime, more mp.. etc, and you get 2 axes.. there is always a good side to that (although, doesn't really improve DoT until you get to 50 for duel wield II). If you sub whm, you'll get 50mp? lol and you can only cure II yourself twice.
To be honest.. i remember a few parties i've had at level 30, where a WHM/WAR was invited and decided he wanted to tank. I'm not naming names- but.. some subs are ridiculous, WHM isn't as bad as you subbing BRD or BLM though.
Take the time to level NIN, i don't think you will regret it, and you will get more party invites! And more exp... possibly. Providing that WHM in your party isn't subbing WAR... or the BLM doesn't have DRG subbed again.