Well usage stats and meta will decide what is valid obviously…
a 5 stoker attack on EV section is a combo, however, 4 stokers and a Engineer would likely be considered more valid…
Basically, giving unlimited access (even if only 3 Mercs per player) means you are able to run with the BEST combo on each objective on each side.
On white chapel a 5 man team on defense can run 4x Skyhammer + 1x Medic on the EV objective and then switch to 3x Sawbones/Phoenix + 1x Stoker + 1x Random Merc. Then when sides switch they are able to go 1x Proxy/Bushwhacker + 2x Medic + 2x Fagger/Thunder. Because you can take the best (or what is considered best in the current meta) at each point the number of valid combos is actually reduced. The meta becomes set for long periods of time and the game starts to feel like you are doing the same thing over and over again…
When you compare that to a Draft mode where each team picks 5 Mercs that they use on BOTH sides you end up with more interesting combos being used. This is only increased once more Mercs are added and pick and bans are added. Because all of a sudden you need to make the choice between 2 Medic on the last defence objectives or 3 FO on the EV section. You know you are likely going to need to take a Engineer… Or do you? Where do you fit a Fragger or Thunder into the mix? Do you sacrifice a FO for one or maybe even a medic?