The current rule hasn't achieved its goal
The original goal of the rule forbidding teams from trading draft picks was to prevent small-market teams from trading away their picks solely to avoid paying the escalating signing bonuses amateur players were receiving. A secondary reason was the fear that it would lead to John Elway-style situations, where a player expected to go at a certain spot in the draft would force a trade to another club, either for personal reasons or to extract a higher bonus. Whether or not this reasoning was valid at the time, it is clearly not valid now.
Small-market or just plain cheap teams selecting near the top of the draft are hostages of their situations. If the best player on the board wants a bonus well above slot for that teams' position, and they are unwilling or unable to pay, they must select the best player on the board whom they can afford, but in the process they have no way to recover the value they lost from having to bypass the best player. Allowing these teams to trade picks would mean that, for example, the Pirates, picking fourth last year, could have received value in excess of what they actually got (selecting Daniel Moskos, who wasn't one of the 10 best players in the draft) by trading their pick to a team that coveted Rick Porcello or Matt Wieters, two high-slot players still available for the Pirates' pick.
As for players and their advisors manipulating the draft, we all know that happens anyway. Porcello didn't fall to the Tigers at the 27th overall pick last year because of draft board oversights in 26 other draft rooms. Scott Boras has established credibility through his success at obtaining big bonuses for his premium clients and a demonstrated willingness to hold players out for a year and return them to the draft pool -- and as a result, his clients typically go in the draft well below where they'd go if picks went in straight order of talent. Many talented high school players have fallen due to high bonus demands and strong college commitments, only to sign above-slot bonuses before they matriculate in college. The draft is manipulated every year, and small-market teams are clearly not benefiting.