ultralights are considered as recreational aircraft and the flight time logged in them does not count towards any professional pilot certification requirements.
A PPL is considered as a recreational licence i.e no commercial activity other than flight instruction so how would a PPL holder claim hours towards a commercial licence if that was the case?
Under the JAA there was a clear statement that Microlight time could not be counted towards aircraft experience however; that changed with EASA who amended the recency and revalidation requirements just over 2 years ago:
We here at EASA tell you (in our regulation AMC1 FCL.140.A; FCL.140.S; FCL.740.A(b)(1)(ii)) that you need to revalidate your license every 2 years and in that same regulation give you instructions how to do it.
We are now providing another way for you to do it. You can count hours in aeroplanes and sailplanes that are Annex 1 aircraft and under your countries National rules ((a)Historic aircraft, (b)Experimental Aircraft, (c) 51% kit built aircraft, (d) Military aircraft, (e) Ultralights under the old 450 kg limitations) or Ultralights up to 600 kg with our new rule Article 2(8) if your country has agreed to allow it.
That means they have accepted Article 2(8) which the UK has.
If the hours are acceptable for revalidation, why would they not count as experience for future licence issue. I had a friend who built his hours on a Rollason Turbulent, the Sirrius would run rings around that in terms of performance.
Whilst there is no clear answer to the original question, the latest amendment shows a clear intent to recognise hours that indicate an appropriate level of experience.