As another vendor (but not kilt related) I too have to stand in defense of having some things drop shipped instead of keeping the entire product line in stock.

In our case, we try to keep all of the repair parts in stock, things that are small enough to fit in a bin (or in a few cases a large box).

I simply cannot afford to keep every single body panel in every single color in stock. I only sell a few a year and they would chew up a large amount of warehouse space for very little return -- some colors might sit on a shelf for years before there is a call for them.

That said, I won't charge a customer until I've contacted the importer and made sure the item was in stock and available to ship right now.

(I'd love to be able to sync up to the distributors and get their inventory levels so that I could reflect them -- but they are almost in the electronic dark ages.)

My feeling is this:
If it is your 'bread and butter' then yes, you should have the items in stock and be ready to ship them immediately. If it is an extra item that you wouldn't be able to carry otherwise, drop-shipping is fine -- but let the customer know.