Re: (Nutaku) Milenium War Aigis (English Version)
On those type of maps I use my three rogues for earlier response until I can get my soldiers out. I wouldn't exactly drop either Elaine or Kerry in that type of situation.
While obviously an option, and the intended purpose of a thief unit type, it can be considered preference map depending. Kerry, I'd agree with she's too pricey and most people can't CR her easily. Elaine there's an argument to be made on at least.
Something to think about: 3 thieves vs 3 valks:
The thieves are going to get out earlier, there's no contest. And they can hold their own vs anything but armors for the most part.
Here's the difference in running the Valks, though 2 normally suffices.
The Valks are going to boost the Unit Points quickly in most rush scenarios.
Their other benefit is there increased Withdraw, to put out bigger units faster.
Something a thief is not good at accomplishing. A thief simply doesn't give much on withdraw unless your using a plat+ or non-CR'ed thief. In some cases they merely delayed a bit, but still don't prevent being overwhelmed.
So you are going to have the pros and cons for both, thus why I said some of it will be preference. I believe the wiser choice is to combine the two options and use both if available.

Still working on Betty myself (never did get her fully CR'ed), although I do have Harrisa fully CR'ed (4 cost, not CC'ed yet).
Im not saying some defence on a duelist is bad, but a defence skill on a duelist is garbage. What unit are you going to tank with elaine that ~20% extra defence is going make a difference on? Its like that male HA with +attack skill, you just sigh every time their skill is up. Sure, if the unit has amazing base stats like a princess or a bandit you could get away with a crap skill. Her one advantage is that she is easily CR;d, but would you not rather save the copies for something else?
Okay, got ya there, misunderstood your comment.
Ya Def skills are a bit lack luster, though they can come in handy on occasion, typically if the unit is under leveled. (Lilia is that way for me, as she's the Def based princess.)
As to saving copies...I suppose you could, but it depends on the person, do they only their gold for units, or do they spend. Sure you can get a better valk that way but it cant be counted on. (and yea a x5 or x10 button would be nice)
Depends on a person patience I guess.