I've got the Steam version of both TS10 and TS12, and although it is true that Steam versions lag behind in patches compared to a standalone version which you can patch manually, as is often the case with Steam versions of things, I wouldn't worry about it too much, it's not the end of the world.
As you have no doubt seen, the Steam version runs okay and there's only the odd (in fact fairly rare) issue with some add-on content which requires the most recent patch, but in any case I can tell you that I've bought stuff like the payware Duchess and other bits and direct from Auran, and Jointed Rail and RRMods payware locomotives too, and I have had them running in a Steam versions of TS10 and TS12 with no problems, so I know for sure that you can do it even without all the latest patches.
Having said that, I did also buy TS12 from Auran as well (so yes I have TS12 twice LOL) and in comparing the two versions, Steam and standalone, since I am in a position to do so, I would say that it is not the disaster many would claim it to be to have the Steam version, it's only really when you get heavily into making your own content that you might find you'd prefer a fully patched up version of it so you have all that is available at hand in terms of DLC bits and pieces. But if you've only just bought the thing, then you probably want to play around with its default content before delving into the creative side of what it can do, so it's really not a big deal, at least for a good while.
Al