Many people have the mistaken idea that the twelve disciples of Christ were not given the gift of the Holy Ghost, and that, in fact, no one was given the gift of the Holy Ghost during Christ's mortality. This is not correct, and once we first realize this we see it all through the scriptures.
First off, the scriptures state directly and specifically that the Holy Ghost is given to those who diligently seek him not only in times of old, but also in the time he would manifest himself unto the children of men.
1 Nephi 10:17 And it came to pass after I, Nephi, having heard all the words of my father, concerning the things which he saw in a vision, and also the things which he spake by the power of the Holy Ghost, which power he received by faith on the Son of God—and the Son of God was the Messiah who should come—I, Nephi, was desirous also that I might see, and hear, and know of these things, by the power of the Holy Ghost, which is the gift of God unto all those who diligently seek him, as well in times of old as in the time that he should manifest himself unto the children of men.
We also find these verses in the new testament:
Matthew 16:15 He saith unto them, But whom say ye that I am?
16 And Simon Peter answered and said, Thou art the Christ, the Son of the living God.
17 And Jesus answered and said unto him, Blessed art thou, Simon Bar-jona: for flesh and blood hath not revealed it unto thee, but my Father which is in heaven.
Christ sends the twelve out as missionaries during his mortality. But what is our doctrine about teaching?
D&C 42:14 And the Spirit shall be given unto you by the prayer of faith; and if ye receive not the Spirit ye shall not teach.
Note that the spirit isn't given by the prayer of faith to just anybody. It requires someone who has the gift of the Holy Ghost to be able to have the spirit come that frequently.
And what about the over all message of D&C 50, which is a reproof for trying to teach the gospel while hearkening to a spirit other than the Holy Ghost. It teaches that the whole idea is that teachers are to teach by the Holy Ghost and those they teach are to receive by the Holy Ghost. That can't happen if missionaries aren't given the gift of the Holy Ghost.
Teaching without the Holy Ghost isn't teaching the gospel.
Joseph Smith put it this way:
"All are to preach the Gospel, by the power and influence of the Holy Ghost; and no man can preach the Gospel without the Holy Ghost." (Teachings of the Prophet Joseph Smith page 112).
Now the Savior does promise the comforter in John 14-15, and there is the day of pentecost when we see the Holy Ghost poured out. But if we just turn to 3 Nephi 19 we can see that among the Nephites as well, after the resurrection they had to be given the Holy Ghost again. It doesn't mean they didn't have it before. And there were some dramatic experiences in that case even more dramatic than what we see happen in the day of pentecost.
There is also one more issue, which is that there are degrees of receiving the Holy Ghost. I have not yet received it in the sense that the apostles did on the day of pentecost, even though I was baptized many years ago and I think the same is true of most of the church. When we prove more worthy, we will not live so far beneath our privileges. And when we look back from there, I think we will see that we had hardly even received the Holy Ghost at this point. But that doesn't mean it wasn't give to us.