Kitchen Designer

Many, if not most, architects are architects/designers/decorators. In my opinion, it is best to work with a single person for a seamless vision. Also, why pay a kitchen designer if it is already included in architect's price.

If your architect is experienced and has done a ton of kitchens in various sizes/local's , then I would go with him.
Otherwise, I would go with a designer. They know best on fit/finish/materials/best location of various sizes on cabinets. In general a specialist is best when you have a unique spot like a kitchen.
We used a designer on ours, there are many details involved and a general architect just may not have the knowledge .

If you are doing full shop built custom cabinets, and your architect is skilled in this task, then there might be no particular need for a kitchen designer, unless you have a McMansion sized kitchen that needs special planning. Even then, the cabinet maker is going to do its own drawings and probably is going to have some of his/her own ideas for arranging drawers, inserts, pull outs, drawer closers, etc. If you are doing high end factory "custom" plywood cabinets (Plain and Fancy, Rutt, Woodmode, etc.) or European style (SieMatic, Henrybuilt, etc), the cabinet company reps offer their own design services, it won't cost you anything more, and they will be very familiar with mixing and matching cabinet and drawers, door styles and colors, and adding details that architects won't plan for, like where to put the cutting boards, knife drawer, spice pull-out, drawer inserts, etc. Having built a new house and gutted our apartment in the past 6 years, we did both kitchens entirely separately. The architects drew up basic layout and plans, but the kitchen folks specified the cabinets and final plans, and they delivered and installed the cabinets and counters. There is an awful lot more to a kitchen than laying out some cabinets. It can make your head spin.

Most people don't hire architects for a kitchen unless they're doing extensive work that may require building permits. Kitchen designers or experienced contractors who can make the detailed plans are sufficient. I've done 4 kitchens without an architect.