I believe this change to bases sizes on the marines is a good thing. As a Space Wolves player, all my infantry is in diffenrent dynamic poses and weapons tend to reach way beyond the base. Getting a unit in close combat with another unit that also incluse dynamic models while fighting between two buildings can be very frustrating because they all get tangled up and it can begin to get real messy when you need to remove THAT exact model from the middle of the cluster**** pile because it died.
I was just discussing that issue with a friend I play often with that just increasing th size of the bases on the bigger infantry models could make it easier to get those base-to-base you want without models getting stuck together.
This is clearly not a cash-grab move, as no one is forced to rebase their army and, honestly, there isnt that much profit to be done just by selling bases, especialy with all those third party offering all those nice textured bases.
Personally, I will rebase my entire army. Mostly to reduce the cluster****s of big melees in tight areas. Also, I will use that opportunity to make cooler looking bases as my skills improved a lot since I first started and my old bases are ratter dull.
The only annoyance here is the time needed to rebase an army, because bases aren't that expensive.