It's the whole mentality that if one has limited funds to have a "pro" do only one of the three stages of production, that waiting until the last stage is always the best idea that I just don't get. It's almost never the best idea. If anything, get the tracking done right and the stuff will practically mix and master itself.
G.
If one has limited funds obviously they have to cut corners somewhere and the decision is going to be based on priorities and total cost.
I don't disagree that having a pro involved in ALL stages is helpful. I would argue that the most important stage to have a pro involved is pre-production, before going to the studio. A good producer can recommend changes to the song structure, changes to make the song appeal to a wider audience, and in general provide recommendations that will make the tracks and performances better before a mic is even chosen. But how many budding artists want to pay for a producer? What is the cost over other parts of the production process, and what is the final product in hand after this stage? This is likely the area where most financially challenged musicians will sacrifice first.
What are the costs of tracking versus the other parts of audio production? Whenever I've was involved with an album this is usually the major cost of putting out an album other than promotion. Is it better to have a pro with pro equipment track? Absolutely! There can be some advantages in tracking at your home studio though. For example tracking vocals can be very personal. I often hear from artists that they would never get the "take" that they got at 2:00 AM in their home studio if they were in a po studio environment. Likewise instruments that run direct (e.g. MIDI Keyboards, samples) can be recorded pretty much as well in a home studio as they can in a pro studio. This is where some cost savings can be had without affecting quality. When an artist has a "reasonable budget" I always recommend that they record at least the basic tracks in a studio with a good room. In fact any instrument that uses the room as part of it's tone and presentation should be tracked at a good studio.
Is it advantageous to have a pro with an SSL, tons of outboard gear, and years of experince mix? Of course. The cost at this stage is likely to be less than the recording stage if both were done done at a pro facility. If the tracks were recorded completely in a home environment this is also where the final product is likely to see the biggest sonic improvement (as Glen suggests). However the cost of this stage is still more than ...
Mastering. With mastering there can be some very significant improvements in both mixes done at a pro facility and for home recordists. The cost for mastering at most facilities is less than any of the above stages of production and when all things are considered is likely less than the cost of a reasonable DIY setup if only performing this for a few albums.
So I guess that my point is that many look to mastering not because it is more important than any previous stage, but because it provides (hopefully) the best improvement/cost ratio of any stage in creating an album.
Of course it stands to reason that if everthing one has produced up to the point of mastering is crap, all that you will have after mastering is better sounding crap.