I find that in most cases I can provide useful trace over a serial port.
Sure, you can't write a major War And Peace treatise on every little variable change - just terse status indications to identify key progress points.
eg, if your systems uses a dial-up modem, you might want something like:

Off Hook
Dialling
Ringing
Answered
Negotiating
Connected

and providing that sort of trace in that sort of application shouldn't be an excessive overhead.

Personally, I would tend to be a lot more terse - execution time aside, all those strings eat code space!
Mine would probably be more like:

Hk
Dl
Ri
An
Ne
Co

As previously mentioned, it's good to have a "budget" for debug; and 10% shouldn't be too much - 10% of CPU time, 10% of the code size, 10% of the IO pins, etc.
10% is no magic number; maybe 5% would be fine - it's just good to have a budget!