There's no intrinsic reason for using one form of naming over another.
However, note that _if_ you assign "channel numbers" to specific frequency allocations, you are *permanently* fixing the utilization of that chunk of RF spectrum. e.g. in going from 15khz deviation to 5khz deviation on FM, you'd have to either completely 're-number' everything, or you have non- consecutive "channel numbers" as you go up the band.
When you (the regulatory authority) "haven't decided" what the minimum allowable spacing between frequency assignments is, or even _if_ the spacing between assignments will always be a multiple of that minimum -- it is *really* difficult to come up with a channel 'number'.[TELECOM Digest Editor's Note: This is an example of how someone screwed up when the Citizens Band radio channels were numbered. CB is allocated the space between 26.965 kc and (originally) 27.255 kc. The 'channels' were 10 kc apart, and there were (originally) 23 channels. (Well, not originally, when there were 8 channels, but in later years.) If you look at the difference between 27.255 and 26.965 as divided in 10 kc increments you get more than 23. That's because the FCC took three spaces in the middle and reserved them for use on garage door openers. So we had channel 22 as 27.225 and channel 23 a full 30 kc later, on 27.255. Then the FCC said they would expand the CB area all the way up to 27.405, or 40 channels, although common sense would imply actually 43 channels if you take 27.405 minus 26.965 at 10 kc increments. What the FCC did, in an effort to 'tidy up' that discrepany was run the channels slightly out of order. After channel 22 (27.225) they created channel _24_ at 27.235, channel _25_ at 27.245, then they had the (already existing) channel _23_ at 27.255 where it had always been, and then by 10 kc up to channel 40 at 27.405. Having those two channels out of order in the frequency allocations did make for some tricky programming of the 'gang switches' (revolving knobs which select the channels). PAT]