eu liessa questão e acho que entendo a diferença entre os dois métodos até encontrar um exemplo estranho:

Defina o estilo da célula de exibição de tabelaBasic, Identificador sejaCélula No Storyboard, codifique como abaixo:

import UIKit

class TableViewController: UITableViewController {
    var items: [String]!

    override func viewDidLoad() {
        super.viewDidLoad()
        items = ["first", "second", "third"]
    }

    override func numberOfSectionsInTableView(tableView: UITableView) -> Int {
        return 1
    }

    override func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        return items.count
    }

    override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        // either works fine
        let cell = tableView.dequeueReusableCellWithIdentifier("Cell")! // let cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)

        cell.textLabel?.text = items[indexPath.row]
        return cell
    }
}

Muito simples, mas quando mudo otableView:cellForRowAtIndexPath: método para 1, 2, 3, 4 casos, respectivamente:

Caso 1:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)
    cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)

    cell.textLabel?.text = items[indexPath.row]
    return cell
}

Caso 2:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)
    cell = tableView.dequeueReusableCellWithIdentifier("Cell")!

    cell.textLabel?.text = items[indexPath.row]
    return cell
}

Caso 3:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCellWithIdentifier("Cell")!
    cell = tableView.dequeueReusableCellWithIdentifier("Cell", forIndexPath: indexPath)

    cell.textLabel?.text = items[indexPath.row]
    return cell
}

Caso 4:

override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    var cell = tableView.dequeueReusableCellWithIdentifier("Cell")!
    cell = tableView.dequeueReusableCellWithIdentifier("Cell")!

    cell.textLabel?.text = items[indexPath.row]
    return cell
}

Caso 1, 2 (não funciona):

Caso 3, 4 (funciona bem):

Como explicar? Eu acho que realmente ajuda a entender esses dois métodos de outra perspectiva, qualquer opinião é bem-vinda.