Yes, the question of early exercise is one that can be tricky to grasp. It turns out that it is never to your advantage to exercise a call option early with the possible exception of collecting a dividend (and even that is sketchy).
To use your example, if ABC is trading for $75 then the $50 call must be trading for at least $25 (the intrinsic value). Let's say it is trading for $25.10.
If you sell the shares for $75 and pay $50 by exercising the call, you'll make a $25 profit. However, you would make $25.10 by selling the call to close. Whenever you exercise an option, you surrender all time value and only collect the intrinsic value. So as long as a time value is present, you're better off by closing the option, not exercising.
Of course, you could assume that if the call is that deep in-the-money with a relatively short time to expiration, it is possible that the call may trade at parity for exactly $25. In that case, exercising early still wasn't a benefit (it didn't hurt, but it wasn't a benefit either).
The argument against exercising early assumes the investor is going to exercise the call and hold the stock. In that case, he would receive less money ($25 vs. $25.10 in the example) but is now holding all of the downside risk. In addition, he is also missing out on interest he could have earned by keeping his exercise money in the bank.
Your example is not a true exercise but, instead, a synthetic sale. It's actually a technique called "exercise and cover" that is used if the option is bidding less than intrinsic value, which can happen near expiration.
For example, if the $50 call was bidding $24.90 and asking $25.50 then you'd definitely be better off shorting the stock and exercising the call. But again, that's a roundabout way of exiting the position instead of exercising to actually buy the shares.
However, there is another really good reason why it would probably not be a good idea to short the stock and exercise if you truly believe the stock was going to fall back to $60. In that case, you would short the stock but not exercise. You'd be short stock + long call, which equals a synthetic put. You'd lock in a $25 profit for sure -- but might make more.
If the stock price falls to $60 as expected, you'd close the short stock position out in the open market for a $15 gain. The call must be worth at least $10 but would now definitely have time value so you could close the call and net far more than $25.
Even if the stock price never fell, you'd receive interest on the short sale proceeds and still be better off than closing it for $25. That's why it would be difficult for the $50 call price to bid below $25; arbitrageurs would buy the call, short the stock, and earn risk-free profits.