It does not matter, you can keep the made form many patches, as long as they touch (thst first order continuity), and align (thats second order continuity) and have equal rate of change (this is called 3 order of continuity wich is important for car designers etc..) it does not matter. And no you will have many parts but than again even a polygon many parts.
You see its impossible to make a human out of one nurbs surface, but if you rethink the world it suddenly is.
For example let us take a polygonal model you think its actually on object that keeps together, but form the computers point of view thats not entirely true. Its a illusion, caused by the fact that many strips coincide perfectly to one another. Now this a bit of mind trickery, it depends on how you think of things. The tehrm one object is very loose, form a certain point of view you could say a pencil is one object thats what your brain classifies it but in reality it is probably 3-6 depending on how you think about it. Now maya does not really define word object the way you think, it does in fact not define it at all, it just defines the word node all esle is also just a classification illusion.
The answer is hard mainly because it depends on so many factors, first of if you have a continuous patch by all means join it together unless theres also a corresponding patch to match one the other side of the patch network that needs to be joined. Or if you use trims theres different set of rules.
But mostly it depends on what your going to use for rendering the end result, it will eventually be disassembled to polygons for sampling but at what stage depends on what our production pipes rear end looks like. If its going to be a tooling machine or a micro polygon renderer with reyes algorithms or similar definitively keep it nurbs if you can manage the patch border without tearing. On the other hand if you use some other kind of rendering algorithms that rely on pre polygon generation you might as will polygonalize it anyway, that is if you can mange the load.
heres one example that will defy what most people thin think of objects (that is if you didn't know this already), paste into command line and run:
CODE
{
$sph=sphere;
move -r 0 1.5 0;
makeIdentity -apply true -t 1 -r 1 -s 1;
$pln=nurbsPlane;
$cn=polyCone;
move -r 0 -1.5 0;
makeIdentity -apply true -t 1 -r 1 -s 1;
print $sph;
$s=listRelatives -shapes $sph[0];
parent -add -shape $s[0] $pln[0];
$s=listRelatives -shapes $cn[0];
parent -add -shape $s[0] $pln[0];
delete $sph[0] $cn[0];
}
Now see nurbs and poly in one object 
Ok i cheated its just because most people have a limited sense of whats there.
