Well, the -r- seems to be more indicative of perfectivity compared to -v-. Also less confusion between -r- and -b-:
1st conj
amav-i-t
amav-er-a-t
amav-er-i-t
2nd conj
monu-i-t
monu-er-a-t
monu-er-i-t
Even the 3rd conjugation doesn't even have -v-
leg-i-t
leg-er-a-t
leg-er-i-t
4th conj
audiv-i-t
audiv-er-a-t
audiv-er-i-t