We should only accept this if we are dealing with AC and even then it is with frequencies above anything really meaningful like over a few hundred kHz that this would even matter. DC exhibits no skin effect so the current flows through the entire cross section of each strand. So no matter how thin or thick it is or how many strands or how much surface area it has, the current flow is the same. All that matters is the total cross sectional area of the cable. To give you an idea of how this is a non-issue, the skin depth at 50 Hz AC is 8.5mm from the surface to the core of a solid conductor. At 50Hz you'd need to be using a solid conductor over 17mm (5/8") in cross section area to see any actual impact from skin effect. Stranded wire of the same gauge, even if it was only three strands, would fix that instantly until you get each individual strand back above 17mm in diameter.
The only purpose for using more, finer strands in a cable in a DC circuit is to increase its flexibility. It does nothing else for you at DC. All wire gauge measurements (such as AWG, MCM, mm2, etc) base their sizing off the sum cross sectional area of all strands in a given cable/wire. It's just that AWG doesn't make this apparent with the number assigned to each gauge, whereas MCM and mm2 are the actual cross sectional area.
As an example, this is why 10 AWG stranded is often nicked or has several strands cut when using a stripper meant for 10 AWG solid (a common folly since most strippers are made for solid wire). 10 AWG solid is a smaller overall diameter than 10 AWG stranded due to it being as compact as you can get it, while the corresponding stranded cable (if its truly the actual AWG it claims to be, most cheaper cable isn't) has a larger overall diameter since there is air space between the strands. Every time I use my strippers on stranded wire I end up using the next size up since they are designed for solid AWG, unless it's cheap stranded wire that is using the diameter of a solid wire to claim what AWG it is, then I have to use the corresponding cutter on my stripper.
This brings up an important point on why to avoid cheap stranded wire. If it isn't actually copper clad aluminum (which is a poor choice due to higher resistance than copper and very poor oxidation resistance for the alloys used, hence the need to clad it in copper), the odds are it's not the AWG it claims to be but is probably closer to the next size down.
That being said, if it's a non critical setup, or you're not trying to match the gauge as close as you can to the running amps because of installation issues, then the cheap stranded is fine, even if its not exactly the gauge it claims to be. Though be careful because there is cable out there that is so far away from being the gauge it claims to be that it's actually several steps down in size. But stuff from the auto parts store is generally fine, or at least close enough. You only need to be concerned if you're designing to the edge. Bigger is always better, so going up one size is usually a good option and generally doesn't add much to the cost overall.
