There have been plenty of very smart defenseman who have struggled to play proper defense despite their best efforts. It's not just about a willingness to improve, a lot of it is instinct that can be very hard to overwrite. And assuming you're able to do so, there is the risk that the defensive adjustments needed to be an NHL player significantly handicap the player's offensive game. Parekh makes a living with his ability to walk the blue line and exploit space in the neutral zone - both areas where he'll be asked to make nontrivial adjustments to his decision making.
There can also be aspects of a player's personality that can cause issues - Parekh has all the talent in the world but is extremely soft and passive, is that a switch he'll be able to find? Some players are never able to bring the necessary intensity to their defensive game because its just not in their nature. Dougie Hamilton is a great example of this.
I don't think any of this means that Parekh is condemned to be a weak defender for his entire career, but there are valid questions that he will need to answer that have nothing to do with his willingness to improve.
I have similar concerns with Buium, but these issues stand out much more with Parekh IMO.
Sometimes it is just a lack of willingness to really improve it. Dion Phaneuf is a great example, as both Regehr and Warrener wanted to teach him to play better in his own zone, and his reply was to the effect of: "I get paid to put up points and blow guys up, I don't need to learn defence". Some guys will figure out too late that they need to put a more complete game together.
Also, you really believe he is soft? He is a complete rat out there egging on the opposition. Soft and passive are not words that I would assign in describing Parekh at all. He has a bit of a mean streak in him.
Conroy was asked about Parekh's defensive game, and he point blank said that first and foremost, this kid's offensive game was not something that the Flames want to stifle. Given their track record lately with other offensive-first defencemen, I will bet that they round out his defensive game. Parekh has mentioned this several times even ahead of getting drafted by the Flames, and when you look at his play in the CHL, he really isn't even a quarter as terrible as most people lead you to believe.
The proof will be in the pudding, but it is always - ALWAYS - easier to teach a prospect how to play passable to decent defence than it is to teach offence. The kid needs to have a bit of a brain (which Parekh seems to have), the willingness to learn it (which Parekh thus far seems to want to do, though we will see how strong that want really is as time goes by) and the right environment in which to teach it effectively (based on the Flames already having a bit of a history at it with some other players, that's probably just as good as an environment as one could hope for).
Time will really tell how good Parekh really is (or becomes) at both ends of the ice, but he does seem to possess the required attributes already (size, skating, IQ, willingness, personality and environment). Doesn't necessarily mean squat becuase it could just be lip-service at this point, but I do believe at this point in time, he seems both willing and capable of being at least decent defensively.