I find this a commonality as well. Its a difficult exercise to squeeze into a short class period so I think it gets glossed over a lot. It can be tricky to find enough qualified uke who don't mind revving it up some. Also, only one person can be nage at a time...hence the extra time consumption. Still, its a great equalizer. Often I've witnessed shinsa where the testing candidate displays excellent technique with his parters in one on one situations (especially predetermined uke), but start to fall apart, or at the very least display a noticeable dichotomy in skill, when it comes to randori (usually, in tests I've seen...the candidate has known uke for all requirements but whomever gets there first is uke in randori...especially when your talking 5+ person randori). It definitely is one of those things that could be practiced more. I recently had a discussion about this with my teacher.
Sometimes during practise we are taught to restrict ourselves to three particular techniques. The first 45 minutes we rehearse those three techniques and for the last 15 minutes we do randori based on those particualr techniques sometimes in groups of three or four.
Sometimes it helps to focus on various weaknesses during randori.