Base Gun Build

When it comes to building up a .45 wad gun, everyone turns their nose up on the inexpensive models like the Rock Island Armory for a base gun, what's the reasoning behind that?
I am thinking of taking a dip into the world of 1911 gunsmithing and thought I'd experiment with an RIA I have. I figure I bought it for $400 so if I mess it up I won't feel as bad as I would if I screwed up a Springfield or Colt.

Your thinking on minimizing the dollar risk is sound.  I have used RI guns for platforms for some of my experimental guns.  They are not as carefully machined as some others and may require some extra hand work.  The primary negative is concern about the level of heat treatment and therefore how long they will hold up before they loosen up.  I have had the slide-frame fit change in a few thousand rounds.  For an experimental platform that is ok.  For a wadgun you want to use for 30,000 rounds it isn't so good.  Others may have had different experiences.

You have given some thought to what you want to do with the pistol.  That is a good start.  

I have no experience with the RIA pistols.  There are alternatives that may result in less expense and bad language, and better results.  Below is a thread which discusses some:  
https://www.bullseyeforum.net/t1977-rock-island-arms-tactical-1911?highlight=Rock+Island

Once I used an old beat up Ballester Molina as a test bed.

You say you're planning on experimenting with gunsmithing. Since about the only things you'd use from a RIA in a proper build are the frame, slide, and pins, why not just get a frame and slide from JEM, Caspian, or STI, and build it from the ground up with quality parts?  This would eliminate any concerns you might have with using the RIA frame and slide.

I guess the biggest reason is I'll have to go trough the registration process to get the frame.
I already have the investment in a base gun that is just sitting in the safe, so now I can spend some bucks procuring the necessary tools do the work.
I'm am just interested in hearing the different points of view regarding such a cheap (inexpensive) gun as a starting point.

If you're trying to learn how to do something it would be advisable to know that your "base" is true and solid.  Not knocking RIA or others, but if things are out of spec you're going to be chasing your tail. That isn't learning, that frustration.  And because you don't know what you don't know.  You'll not know if something is wrong and you're correcting it.  Or if it was wrong and you're making it worse.

K.C. is right. First define what your goal is, to become a good shooter or gun smith. If it's to be a good shooter do not waste time trying to make a good gun, you will be better off taking the gun to someone that has experience making a bullseye gun and use the time learning to shoot a great score. You will also find that it's cheaper to get a good gun built than to built it yourself if you don't have the tools and knowledge to do it. "Chasing your tail" as KC put it will cost you time and money.

Whether you do a full build or decide to work on a gun you already have, you'll need quality measuring tools such as an outside micrometer, pin gage set, gage block set, etc.  Also recommend a groove micrometer.  Removing material is relatively easy once you know precisely how much and exactly where to remove it.

FWIW my first attempt at a full build was with a Foster frame and Caspian slide.  The slide-frame fit ended up too sloppy for a wad gun so I saved the slide for another build and used the frame as a dedicated Marvel lower.  Learned a lot along the way and I compete with the Marvel/dedicated lower.

Subsequently took a build class from a Bullseye smith...this gave me a profound appreciation for what it takes to build a true X-ring gun.  Worth every penny.

Wishing you success!