Say for example an inexpensive solar calculator or other such similar device with what appears to be lets say a single 2.4V 6uA (microwatt) panel? Is an MPPT used to get the most panel rather than using the panel directly? Is there another solution for this class of panel?
When you're talking about "inexpensive" consumer devices like solar powered clocks and calendars the answer (in general) is no.
However, there is a new generation of energy harvesting devices that are very high efficiency designed to extract much more of the energy from devices like a small solar panel.
You can see an example of such a device here: TI Energy Harvesting Device
You have to balance:
- Price of the MPPT circuitry (which is not that cheap) versus...
- Price of a larger solar cell
For a solar calculator, which draws very little power, the extra solar cell area required is likely much cheaper than a smart MPPT circuit.
For other devices which use energy harvesting, like a dynamo-crank flashlight, this is also likely... because you're going to buy the whole thing for $2 on aliexpress or in a supermarket, which will purchase it at a cost of about $1 from china. And their manufacturing cost does not change if you got to crank it a little harder because it is inefficient.
Now if you have $200 worth of solar panels, then it is extremely likely that the addition of a MPPT circuit worth a few $ in quantity will be highly profitable.
To add to @peufeu's answer in addition to cost issues there is the fact that MPPT can only improve the efficiency by a limited amount - say 10-20% and it takes power to run. If it takes more power to run than the additional power it provides there will be a net loss.
With high power arrays the overhead of the MPPT circuitry is acceptable, I doubt if you could run any MPPT circuitry on the 10uW or so you mention in the question.