Prior to the 1.7 sprint key, PvP was a slightly more skill-based in my opinion. There was a wider gap between the "pros" and the inexperienced players.
Aside from the fact that a huge amount of people used BSM...
Being an exceptional sprinter was a legitimate way of excelling in PvP. It helped with your strafing, knockback, and all sorts of aspects of PvP.
The addition of the sprint key really lowered the bar of skill requirement. Double-tapping W was suddenly a less effective - and thus less desirable - option. Anyone who had become proficient with the old style of sprinting was suddenly being overtaken by new players simply because of the sprint key.
It was a situation in which you either had to adapt, or you would sink. Either learn to use the sprint key, or else risk being beaten by everyone else who used the sprint key.
Also, it had another effect. More hackers.
Why?
Here's why:
Many of the people who wanted to "hack" on MCSG didn't want to get banned. That's obvious. So what did they do? They searched for a hack or advantage that would be difficult to detect, but still effective. That brought them to Better Sprint Mod, which was possible the most common disallowed Mod used on MCSG prior to the implementation of the Anti-BSM system. It was also used by BajanCanadian, one of the largest MCSG YouTubers for quite some time. (Him and his buddy were the source of all of the Bacca skins.)
Once the 1.7 sprint key was implemented (as well as the anti-BSM system), they needed another advantage. BSM wasn't cutting it, as it was pretty much equal to the sprint key. And so, they searched for legitimate hack clients, a step above BSM.
While this may not have been directly aimed for hacking on MCSG, as we had the Anti-BSM system, there were other servers with which this concept would have applied, and so they carried their hacks over to MCSG eventually.