Time signatures are important and while 3/4 is obviously 3 crochet (or quarter) beats in a bar is not the same as 4/4, which is obviously 4 crochet (or quarter) beats in a bar. Clearly, they are not the same.
So if you are playing a piece of music that is in 3/4 time, then do not make the third beat in the bar longer by pausing on it . Realistically this is converting it into 4/4 time, or 4 beats in each bar.
Even though 4/4 time is the most common time signature in western music traditions, 3/4 time signature is still a valid time signature and has a completely different flow and feel to it. In my opinion it is worth learning and playing.
I have even witnessed professional musicians try to take a piece of music and presume it is in 4/4, when it is originally in 3/4. I personally find it irritating when professional musicians do this because in my opinion, they are butchering the piece of music. I must admit that I really enjoy listening and playing music that is in 3/4 time.
While I personally do not like music that was written to be played with a 3/4 time signature, changed into a 4/4 time signature, it is purely my opinion. Just like anything else, when it comes to being creative and adapting music to your own personal tastes and styles, time signatures can be adapted, converted, messed with or completely changed if you wish.
What I am saying is that there is more than just one time signature and that the 3/4 time signature is equally valid to learn as 4/4. Do not make the mistake of playing the two time signatures exactly the same. If you deliberately change it, then that is a different story, but it is worthwhile learning both 3/4 as well as 4/4.
I must make note that there are other time signatures as well and I think they all are actually worthwhile learning.
Of course, as always, its up to you what you want to play and how you want to play it.