Can vendor lock-in ever be valuable?

Depending on the situation. It can be in your benefit to negotiate better price, more services and easier to train your team. For example of you are deciding to use only one vendor for hardware vs splitting the purchase from 3 different vendors. This will increase your importance at the vendor that can lead to more benefits down the road.
Now, that is not the case always. For example if your backup vendor locks you i.e there is no free backup recovery tool to use once you are not with that vendor to restore backups from the period that you were with the old vendor than that vendor lock in is very bad.

If you get a vendor at an early stage and they need customers, you can lock into some early bird and really low pricing.

Yes, it can be extremely valuable to the vendor :-)

Valuable? I doubt it, as I do not see an inherent value in such lock-in. Could it have some strong positives and in some situations, at least temporarily, be desirable? Yes. In a longer term, I truly believe that openness wins.