For them to be worthwhile the mana cost needs to be lowered, and either they need to be buffed or the Demigod needs to be able to summon more of them. Although, if they were turned into a passive summon, like Oak and Erebus, then it'd make them a lot more likely to be used.

Personally, I'd like to have double the amount of Shamblers/Yetis (total and per summon). This would increase the usefulness of Entourage for QoT (although, I still think that the +gold ability is a waste of an ability).

Just dropping in to share some thoughts:

For the most part, I find Yeti's to be useless and Shamblers to be fine. Shamblers to pale a little compared to Spirits of Nightwalkers though, simply by virtue of how outnumbered they are.

However, in the interest of science, I did try a Sedna Yeti build the other day in a single player skirmish (Yeah, best testing ground ever. Although giving myself Normal AI's and fighting Hard's probable balanced it out as the Hard's farmed the Normal's). For most of the game, the Yeti's were gimp. They died moderately easily and did next to no damage. Although I used the favor item that gives them knockback, so I could actually clear lanes as Sedna. I took all the passive minion buff skills, and even gave the Unmaker Artifact a try. It was no good, until late game. When I picked up the level 15 Yeti skill (the cleave one) they became really good. Instead of getting bogged down and swarmed by Nightwalkers, they would destroy them all. And by this time, I had around +50 minion damage and +30% attack speed for minions.

It still wasn't a very effective build, but it was interesting to find out that they could hold up in end-game. Apparently that 75% cleave power is what they need to be worthwhile. An interesting change would be to give them 50% cleave damage as an innate power, and then let the skill upgrade it to 100% (along with a damage boost) or something.

I think stacking items that buff minions and getting talents that buff minions makes yetis and shamblers pretty good. Ive played as sedna with a friend against some computers and, being sedna, i couldnt die because of healing, and my yetis and other minions were buff enough to steadily kill other DGs.