There's a significant difference between a bullhorn and television air time. You would be saying that I, as a candidate, could not spend my own money to by advertising time and deliver my message. Just as McCain-Feingold limits when I, as an individual, can spend my money to express my personal view on the election. Both are wrong and should not be allowed.
Once you start the limiting, it may sound simple, but it becomes very complex. Who's limited and when and how? Public funds -- if only public funds can be used, then who qualifies to get them? It's dangerous territory, because freedoms are lost in little steps.