It looks like there have been a bunch of different types of reactions to why this might be a bad idea. Let me see if I can try and answer some of the criticisms:
1) Why is this change needed?
Several reasons that add up to this being the most elegant solution, which is part of why it's hard to explain. Part of it was trying to deal with the problem of players whose salaries exceeded their utility, causing them to become nomads. We'd like to ensure that a player making $400k/week will actually provide close enough to $400k/week of value that they're useful to somebody who can afford them, and this helps us get closer to that goal. At the same time, it's a bit more realistic, it makes the training mini-game more interesting, etc. But it's not going to stop you from making unique players or anything like that. It's also in part a reaction to transfer prices and trying to stabilize both the salary curve (the change for this season was smaller than in previous seasons) and transfer prices for extraordinary players (of many types, both smart and not-so-smart). To put it another way, if you have a favorite player whom you'd like to make mono-skilled, it would be nice to be able to keep training him forever without his really becoming a liability to your team. A change like this is a gentle nudge in the direction of making that possible, too; the problem with mono-skilled players is NOT that they are too effective, but rather that they are not effective enough given their salaries, yet no pool of comparable multi-skilled players exists. Our hope is that this change will bring those back into better balance, giving teams more of a choice of strategies for how to build.
It's a bit complicated to answer this one, because there really were several things we reacted to, not just one overriding concern, and we picked a solution that attempts to solve them all simultaneously. The plurality answer would be the high-salaried nomads.
2) Isn't this going to kill players because they run out of potential?
Let me tell you a little bit more about how salaries and potential work. Both of them are based (in slightly different ways) on the idea of looking at all of the different ways a player could be used, picking the one in which they will be most effective, and giving them a salary / calculating their remaining potential on that basis. The "best position" shown on your player is roughly (again, not quite exactly) an indicator of how the player is currently viewed. If you have a point guard, and he improves in shot blocking, he'll almost certainly still be evaluated as a point guard, which means that the shot blocking will be a neglible difference in how effective he appears to be as a point guard, and thus have a negligible effect upon his salary/potential. To put it another way, what we're doing is helping your trainers to make your players better in a way that increases the ratio of their effectiveness to their salary / remaining potential, and that should generally be merely a good thing.
3) This invalidates my strategy!
There's a good reason we announced this one well in advance, and it's in order to let teams try and plan properly. We try to do that every time the rules might impact a strategy. I don't actually expect an effect on this level is really going to have a major impact, but it's certainly a change, and there are some strategies that are hurt and some others that are helped by any change. That's always true, and larger changes are announced in advance in order to help prevent teams from being surprised by the change.
4) But...I don't believe you!
I'm not sure how I'd answer this one, but I will ask: if you were given the option to avoid skill increases from cross-training (all other effects would continue to apply; you'd just be giving up on increases with zero compensation), would you want it? If enough people actually would prefer that, it would be easy for us to implement...