I have some experience of coaching National and international competitors, as well as competing myself for many years.
We practice drills on the floor as warm up exercises and as part of our skill training programme using locks, rolls, turnovers and shime's, we also practice links from tachi waza(standing) to newaza (ground work)the amount of time allowed by British referees for positive ground attacks is less than 30 seconds and so links are vitally important, the hold or lock needs to be on prior to you hitting the floor in order to secure an effective pin if possible. We also practice probably 50% mat work in a free practice situation, i.e. with a competitive partner.
Newaza accounts for a high percentage of contest wins in all domestic and international events, ignore it at your peril, we also do similar with tachi waza.
Hope this is helpful