It would indeed make the layout easier to learn but Shai felt the advantages of moving it were sufficient to mandate a move. There are explanations on the forum why the S had to move. Snipped from some thread:

"That [swapping s and r back] would make it easier to learn, but it would increase same finger with FR, CR, and to a lesser extent, SW. To fix that, F and/or C would have to move, making it harder to learn and making ctrl-C harder."

Swapping R and S on Colemak doesn't make THAT much difference.  Here are the numbers for my corpus:

TOTAL DIGRAPH OCCURRENCES: 477191

STANDARD COLEMAK
sf: 393
fs: 220
sc: 703
cs: 108
wr: 149
rw: 394
rx: 4
xr: 8
total: 1979 (~0.4%)

COLEMAK WITH S AND R SWITCHED:
rf: 302
fr: 687
rc: 443
cr: 416
sw: 888
ws: 181
sx: 1
xs: 8
total: 2926 (~0.6%)

So, out of 477191 digraph occurrences, you'd end up typing 947 (~0.2%) more of them as same-finger; that's roughly 2 out of every 1000 characters--not a huge deal.  Moving S from its QWERTY position was clearly a refinement, not a major improvement.

The bigger disadvantage of swapping the two letters is that you now have a non-standard layout, so you're responsible for supporting yourself (e.g., you can't just jump on the official Colemak site and download install files for whatever new computer you sit down at.)

As has been stated, it would increase same-finger for fr, cr and sw. Phynnboi's measurements show that switching S and R would increase same-finger ratio by 50%. While that is still a small minority of all typed characters, for those of us who don't much like same-finger digraphs a 50% increase is a big enough deal to swap the two. If it's any consolation, I promise that with time you will get used to it.

On the flipside, you may notice that quite a few people are asking for a version of Colemak without the optimisation constraint of being similar to qwerty. They want a more hardcore layout that has cast of all shackles, and are of the belief that Colemak has made compromises. My feeling is that Colemak isn't really deoptimised. Rather, Shai in a sense picked the layout among many layouts of similar strength that is the most similar to qwerty. Leaving S and R in their qwerty positions would in fact be a compromise that would displease the hardcore crowd.

I've noticed that same-finger digraphs aren't nearly so bad when they end on the home row (as in "FS" on Colemak)--in fact, I hardly notice those.  It's the ones that end on the top or bottom rows (as in "SF" on Colemak) that throw me off, for whatever reason.  From that perspective, "bad" same-finger digraphs increase by only ~10%.  Still not great, but not a killer.  It's still worth soldiering on with the official layout, I think, for the added support.  Alternatively, you could try the "ASSET" layout, which is competitive with Colemak and keeps "S" in its Qwerty position.

It's not so much whether they end on the home row I think? FS on Colemak has that nice downward slide from the upper to the home row, which isn't bad. Take something like CS or KN, and I'd recommend alternative fingering because they're really awkward. Luckily, CS is very rare (I think) and KN is one of the most easily done alternative fingerings.

Even the maker of ASSET thinks that Colemak is a better layout, by the way.  ;)