Considering a Dual SIM phone for 1 number + 1 data SIM. But does it matter?
 

So I need one SIM slot for my number in whichever country and the 2nd slot might be interesting for possible (light) data use only. 50-100 MB is more than enough.

Best deal for data would be a pay-per-megabyte deal, such as Three 321 in the UK, completely flexible: http://www.three.co.uk/Store/Pay_As_You_Go_Price_Plans

But I realize not all countries offer this that's why a 2nd SIM slot might or might not be interesting, for a data only SIM. But these SIMs often not offer PAYG plans, and I certainly don't need gigabytes, so I do not want to pay for that much data.

In your experience, could you use a dual SIM phone for 1 number + 1 data extensively (because of pricing policies in different countries) or dual SIM was not a benefit for data, only for 2 numbers?

Consider the mighty iPhone does not come in a dual SIM version.

I've recently bought one, not really used it a lot yet.

And others are considering it. See this thread about the choice of dual (or more) SIM Android phones

http://www.prepaidgsm.net/forum/showthread.php?t=8112

I recently switched back to a single SIM phone as Wifi and GPS sensitivity of my Zopo ZP980 (dual SIM) were inacceptable. Nevertheless I used the Zopo ZP980 for half a year and while travelling I used it exactly in the scenario you are asking for - while the primary SIM slot would host my German home SIM card, the second was for local SIM cards which gave me affordable data and cheaper local calls. This worked flawlessly as you can select at will which SIM to use for data, voice and sms (the latter can also be selected case-by-case when calling or sendings texts respectively).
Though I cannot recommend Zopo because of their obvious quality issues their phones are special in their ability to assign the 3G-capable radio module to any of the SIM slots from the menu. All the other phones I'm aware of have their 2G and 3G radio module assigned firmly to one of the SIM slots which will probably result in more frequent SIM swapping.
For more details see the thread Andy has already linked above.

My Cubot GT90 allows 3G/2G slot swapping - more common than you might think...

+Steve :)

all dual sim phones i own at the moment allow the slot swapping... seems to be a feature of the current MTK chipset

Maybe I was not clear in the opening post: I do not need the ability to be reached at two different numbers. I do not need the hot-swap feature either.

Why I am considering it is because who knows? maybe in many countries I can get cheaper data on a data only SIM than as an add-on to a mobile SIM? I do not think about Gigabytes here. Three UK's 321 tariff is great for data as well for the UK, but maybe other countries offer these better data deals on a data only SIM? Talking about in the 100 MB range.

As the topic of this forum is international SIM cards and such, I just want to know if it's worth thinking of those phones for this reason, as I did not check all the countries in advance for which this might be interesting in the future (and who knows, where the mobile and telecom industries are heading?).

Perhaps some of you had the chance to check the tariffs for data only vs. data + voice SIMs for this and that and maybe even so many countries and came to a conclusion.

Considering also that you can have a much better selection of single SIM only phones: not all models come in a dual SIM version.

I would say there are three main reasons to use a dual sim phone-

1) use your personal and your business SIM in one phone to avoid carrying 2 handsets and
2) optimize your cost either domestic or on roaming by having a 2nd SIM (e.g. one with good voice tariff and the other with good data tariff)
3) roam with your domestic number for availability and use a local SIM for cheap data/calls.

So yes, your thoughts are valid...