反乱	459613
鎮圧	162133

Yes, it's rather obvious and not that common compared with the parts.
OTOH I put the three glosses into GT and it returned this expression for two of them.

I don't think it's needed. In previous years, all sorts of expressions were added to JMdict but we tend not to accept collocations like this any more. It's A+B (a sum of parts).

I appreciate all the time reviewing my comments.

My two cents, since I guess I started this... 
Since 反乱を抑える and 反乱を鎮める actually were approved for deletion(which was my larger motivation, because I actually found them to be misleading), I think it would be completely consistent to *not* create this entry. (Consistent with Robin's remarks on 反乱を鎮める).

On the other hand, this is a very nice collocation now (and there are lots of collocations in this dictionary, whatever the actual editorial policy might be). If I searched for either 反乱 or 鎮圧, I think this entry would be particularly elucidating to see, and I would probably build a flashcard off of it. (As we've seen, there are a lot of terms for "suppress" to chose from in Japanese, for example). 

Beyond that, I don't think I have a definitive view on whether this should or shouldn't actually be here.  Not that it was my call anyway :) But I won't be bothered with whatever everyone else decides.

反乱を鎮圧する	1191
反乱を鎮圧した	1020
反乱を鎮圧して	329
Eijiro:
左翼の反乱を鎮圧する : put down a leftist insurgency
大きな反乱を鎮圧する〔～の〕 : suppress the great rebellion of
反乱を鎮圧する : crush a rising
GG5: クラッスス - .. 70 BC にスパルタクスの反乱を鎮圧)
3 Tanaka sentences

反乱 （はんらん） (n,vs,vi) insurrection; mutiny; rebellion; revolt; uprising
鎮圧 （ちんあつ） (n,vs,vt) suppression; subjugation
Possibly a bit obvious.