I am actually in the process of boosting a ton of new toons. For me, I have decided to fill out all the empty slots on my current server with the classes I am missing on each account. What I did was during the sale last year, bought 4 vanilla keys for $5 each. I have added these keys to 4 new RAF accounts linked to the old accounts. Then with my booster I am boosting in teams of 4 toons, 2 on exisiting accounts and 2 on the new RAF accounts. This meant on a couple of sets of toons I had 5 sets of 4 to level. Once I am done I will take the odd toon here and there from the RAF accounts and transfer to the 5th account and then cancel the RAF accounts.
This will leave me with all my accounts on this server full, with all toons level 61 or above. Once this is complete I will boost some of these toons up to 65- 67 and use a mix of boosting and quests to hit 70. Then I will quest in Northrend where I will also be able to skip 90% of the collection questions due to not going to quest in Northrend at 68 and also using the BOA Shoulders and chestpiece.
My current state of play on this process is that I am nearly done with SM on all the accounts and about to move to BRD form 40 - 45 and then Scholo to 59.9 and ding on the RAF account to grant a level to the old account and then ding it to 61. I will have to do this for about half of the toons I am leveling, the other half I will bring to level 45 and then use the grantable levels to level them to 61 also.
I could not even think of the idea of restarting again with brand new accounts, losing all the gold, gear and professions.
My two cents