I don't wish to appear to be disagreeing with people like Rob, but I suspect that there are times when the best approach for a skilled professional is not necessarily the best approach for an amateur. Sometimes what works well for someone with the right tools, an enviable level of skill, and years of experience might not be the ideal technique for a beginner.

Having just finished my tenth wooden kayak, I think I could probably class myself as an enthusiastic amateur and don't pretend to have the skill or experience of some of the people who contribute to this forum. For someone with similar skills (or lack of skills) and tools to me, I believe that using both internal and external stems is definitely easier, probably quicker and possibly lighter than not using them.

When I built my first stripper, in spite of the fact that I had internal stems and planned to fit external stems, for some unknown reason I made the joins at the bow and stern as neat as they would need to be if I were not fitting external stems. I am sure that the time it took me to make those joins neat was greater than the time it takes me to fit laminated external stems.

Laminated Paulownia or even WRC internal stems could easily weigh less than the accumulation of glass and resin that a lot of beginners would finish up with in the bow and stern as they attempt to force the glass into the narrow V at the bow and stern. I know some people can keep it neat, but I suspect that a lot of beginners can't.

Fitting external stems also allows me to use hardwood which I assume would help the glass at the bow survive better if I chose to ram a rock or another kayak.

I believe that a builder who is at the stage of asking questions about whether internal and external stems are a good idea is probably someone who will have fewer problems, will actually save time and may even have a lighter kayak if both internal and external stems are used.