Good reasons for using that building style:Get creative instead of spamming megatons everywhere.
Well, Megatons fit really nicely on GEO Corp blocks, because they are of the same size attachment size.
2x2x2 to 2x2x2 fits real snug, not to mention that it casual viewer may have thought the Megaton could have been intended for GEO Corp back when the massive cannon was still in R&D.
The GSO Megaton and GEO Corp blocks make a great pair.
(Now to wait for 3x3x3 blocks!)
While I do find the Megaton on GEO Corp a very convenient and nice-looking building style, personally I prefer overcomplicated designs that are almost guaranteed to use many different corp blocks over the Megaton-GEO setup.
A messy mix of different corp blocks that all achieve the same goal: Making a Tech fly with a specific lift system.