Ok, here goes.
I think that ANY free software should be freely distributed. In the context of mods, I certainly think that it should. Some modpacks, not pointing at tekkit, have a bad reputation, but even they can only make more people want to play minecraft, and thence minecraft mods. I can't see a single really good reason why this would not help a mods reputation.
HOWEVER, my opinion doesn't matter. The mod authors do. If they say they don't want their mod in modpacks, I am not going to go against their wishes and do it anyway. This is the whole principle FTB is based on. Respect the mod authors.
The main reason in the past that the majority of minecrafters didn't play with mods is that, for the less technically inclined user, they were hard to install. Even more so when you started stacking them on top of each other. Mod packs try to change that, they try to make it as easy to play with the mods as it is to play with vanilla minecraft.
Correct me if I'm wrong, but tekkit was, by far, the first major mod pack. Tekkit was what made it really easy to install and play with a bunch of mods. But they did kind of alienate some of the major modders by not first asking permission to use the mods, in their small mod pack. They then grew to have a huge userbase, and still didn't get permission from all the modders. (see Forestry)
Now, in answer to GreenWolf's post, tekkit didn't do anything directly wrong. Like I said, it IS a great idea. But they did it in such a way that it was obvious that anyone could make a mod pack and easily distribute it, without getting specific permission for each mod. Although FTB is still working it out, their private packs are designed so that you have to get permission.
Bottom line, tekkit does something and deals with the consequences. FTB asks the modders first and respects their wishes. It's not a fight, neither way is against the law.(not debating that) Just pick which one you want to support.
Note1: And I'll just say here, I don't dislike tekkit, I might not agree with some of their policies, but I think they were a great thing for the modding community. If they went FTB's path, maybe things would be different, but I'm not going to deny the attention they brought to mods.
Note2: The disclaimer in my sig applies, this is completely my opinion and does not reflect the opinion of the FTB team.
Note3: I know I just went and wrote a big post about it, but honestly, fighting over the issue doesn't solve anything. Pick one and use it. (or both if you don't care about the politics)