More Sketch Driven Speedbumps

[RDG3PO]

I'm trying to assemble a caster with a container (see picture below). I realize there are many ways to skin the cat here in this instance, but the first thing that comes to mind is creating a sketch and mating the axis of the caster to the sketch. As you can see, mating the holes won't work because the holes in the caster are slotted. 
In a previous topic I posted it was determined that the sketch has to be located in a separate part in order to constrain to it. This doesn't make much sense to me as it creates another "layer" that you have to work through in order to achieve the results. 

Can someone explain to me why Inventor is made to work this way, or possibly even offer up a quicker more streamlined way to get this caster on the model where it belongs?

[JDMather]

I would do it pretty much the same as real world equivalent.

I would put workpoint or sketch geometry in the castor plate slot location where the fastener will be.

Some use an iFeature to automate this.

[chad38]

I would put 4 cylindrical surface extrusions in the center of the obround slots on the castor, then use those surfaces to mate to the holes. But that's if the hole pattern you're mating to match the center of the obrounds.