I don't like thumb shifters on road bars for the following reason. If you ride primarily in the drops or on the hoods than the shifters are in an inconvenient location.
I understand what you're saying, but don't find this to be the case. I was actually quite used to downtube shifters -- no issues with reaching to trim shifts, and I was additionally using
friction downtube shifters -- not index. I think riding downtube friction shifters is a great aid to learning how to really "reach around" and maintain good balance.
I seldom ride on the tops of drop bars, but mainly use the hoods and curves of the tops, and in the actual drops -- I ride a Surly LHT so the drops are fairly high compared to most road bikes.
Reaching to the tops to shift is really extremely easy compared to those
If you ride primarily on the top you would be better off with a flat bar.
I don't ride the tops much at all....rarely actually. Flat bars don't have enough hand positions for me and certainly no good ones for getting low in the wind....though my drops are relatively high, so I'm probably not quite as low on them as I am on my other road bike.
I guess what I'm leaning toward is that
I get that this is not commonly done. On the other hand, it seems to feel pretty natural (to me) and seems to beat downtube shifters easily.