Over the years, various games have featured quite diverse recruitment mechanisms and I have no problem with coming up with more of my own as well. =)

I'm mostly talking about soldiers here but the same applies to scientists and engineers, assuming they have an individual skill level at all.

1. Soldiers as wares

This is the bare-bones minimalistic approach that was used in X-Com 1+2 and the Xenonauts alpha builds.
You buy nondescript "Soldier, 7 pieces" for a constant price.

The obvious advantage: it's easy to code and there is no real point in imposing a limit.
It's not very exciting, though, and leads to the annoying "hire & fire" routine from X-Com where you would hire 20 soldiers and only keep the 2 whose stats didn't suck completely.

So while basically the easiest system to operate for the player, it leads to a boring chore of weeding out the most useless candidates.
No fun.

2. The recruitment pool

That is how X-Com Apocalypse, Jagged Alliance, UFO-ET, and many other games handled it.

The player is offered a selection of personnel and can see their stats before hiring them.

2a. The infinite pool

The visible size of the pool is limited to maybe 10 soldiers. Whenever you recruit one, another soldier with random stats is added instantly.
There is no advantage over 1..
The player would just hire / fire until he gets all "good" soldiers, making all the effort spent in creating a more advanced recruiting system pointless.

2b. The limited pool

If the pool is only refilled / added to every day. (or whatever time frame)
The player has to manage these limited (manpower) resources.
Instead of buidling a small team with only the most excellent recruits, he might opt to use the training system and hire some of the clumsy recruits, too.

That makes the game's training system more valuable and allows the player some leeway in his team-building strategy.
Beats the drudgery of hire / fire any day.