yes, it will go into vanish mode...as will G.nodosum, and G tuberosum....and possibly G,aristatum. They are, I suppose, what you might call a spring ephemeral (we tend to not use that expression in the UK)....but are completely reliable, year on year.
Yes, it does take a while to build up a substantial rootgrowth....but is one of the only pratense types I would bother with.

Camps, your advice is always much appreciated. I confess that I have a "thing" for hardy geraniums and I am always on the lookout for more, whether as growing plants or as seeds.

Why do you say that 'Plenum Violaceum' is one of the few pratense types you would grow? Are Geranium pratense and its many forms too robust, too weedy, too ugly...? Even the wild blue G. pratense seems lovely to me and I would love to have a meadow filled with it. I think most hardy geraniums are unobtrusive enough that I wouldn't mind them reseeding all over (the finely cut foliage of most of them doesn't seem to smother and bulldoze neighboring plants).

That said, I am considering starting a batch of 'Mrs Kendall Clark' from seed. Should I avoid it?

Nothing wrong with ordinary G.pratense....although it does tend to get a bit mildewy and has a fleeting bloom season. I do have (lots) of it - I guess I should have said that violaceae plenum (along with himalayense Birch Double) is the only earlyish variety I would buy.
Mrs K C is a real winner at dusk - it gets that moon touched, luminous electric glow.

Great to know that about the color of "Mrs Kendall Clark'. I went ahead and splurged on some seeds from Jelitto, since they offer MKC as part of their Gold Nugget collection. I thought it would be nice to have geraniums germinate quickly and uniformly for a change, lol!

The way you describe the growth habit of G. pratense, it almost sounds like our native G. maculatum (except for the mildew part). G. maculatum is one of my favorite wildflowers.