Those with a CTR, I should think. CTR’s are controlled airspace (C or D, mostly) and flight plan is always required. Even if it be the abbreviated.
Even with my limited powers of observation on a Friday afternoon, I notice that with almost every one of the 36 posts, that the matrix where this concession is usable gets smaller and smaller 
I guess it is useful to people who keep doing a particular flight over and over, and it happens to meet the requirements.
Required or not, why would you NOT file FP when crossing FIR? It is a bit like the flight following in my view – not required but certainly improves the overall safety?
When in doubt – file.
I notice that with almost every one of the 36 posts, that the matrix where this concession is usable gets smaller and smaller.
I guess it is useful to people who keep doing a particular flight over and over, and it happens to meet the requirements.
That must depend on one’s average type/style/category of flight. My upcoming grand tour could be done entirely without filing a single flight plan (BE-DE-PL and perhaps a bit of CZ); yet it will be a very unique trip.
@molder is right, though: filing is a small effort and does add value, so why not?
On Friday I have to fly from Dresden to Straubing VFR.
So I am landing in the same country I take off in, but crossing Czech.
Do I need to file a plan?
Not for day VFR. Even if you landed (at an uncontrolled airfield) in Czech, you wouldn’t need one.
Nope you don’t.
Taking your autopilot to Straubing?
PS: Germany does not require VFR flight plans for international flights. Only if the other country requires a flight plan, you have to file one. The Czech Republic doesn’t require it either for Germany. However, in your case you don’t land outside of Germany so definitely no FPL required. You will stay with German FIS all the time, they have agreements with Czech FIS.
The concession between Germany and Austria is in fact valid.
And between Germany and Czech Republic it should also still be possible, according to this and this
And also between Germany and Belgium according to this recent posting.
However, I have to admit that I would not fly any more these distances without filing a flight plan. It’s just so easy nowadays to file a flight plan that I can’t see any benefit in not filing one.
