I think you’re right, at least some people have that motivation, that it is older than the internal/final sigmas. I would imagine that changing such a longstanding practice is difficult, so I’m sure the added argument that it is only 1 character to learn instead of 3 (considering captials) is often cited—although, until the lunate sigma overwhelms the publishing houses, it seems more like 4 characters to learn instead of 3. 
The lunate sigma used to make my eye stumble when reading, but it doesn’t bother me at all anymore, except it still seems really strange to me when it is capitalized. I sometimes like to write out verses in capital letters with an iota-adscript, but the iota-subscript is still, for me, an important crutch for recognizing the dative case—although I suppose if people started writing it out more often, I would become accustomed to that as well.