even if you have nothing to transmit to the outer world, then you can at least use the UART to output some helpful status flags, a huge help when debugging!

I do not consider that, my stance: no ICE, no debug. However, if what you are making does not have critical timings, maybe a serial debug would be acceptable. An ICE may be expensive, but once you have bought it, you have it forever. I have, over the years, collected quite a few. If I have to use a non-ICED chip, I used to prototype with one for which I have the ICE. e.g. I can debug a board with 2 Winbonds with 2 RD2 ICEs with no ill effect. Today, of course, there are the SILabs chips where the investment in the JTAG "ICE" is $99 and cover the entire product range, so, were I to design something, where the cost of the ICE mattered significantly (small volume production) I'd use SILabs if no chip for which I have the ICE would fit better.

Erik