As with nearly all things, each case has to be calculated individually.
There are still companies out there (and some of them are quite large companies) that still use antiquated DOS based systems or something similar. This is entirely because the IT decision of the day was based purely on a 'let's keep the costs down' mentality.
We can discuss 'real world' issues till we turn blue, but the decision has to be made on a case-by-case basis.
Having said that: It is my experience that a single roll out should be implemented. In theory, organisations are already depreciating costs of hardware, and therefore the business case for upgrading is made on the balance sheet (I love using that on Finance Controllers).
With solidplanning and a good team, a company up to 600 should be able to roll out in 6 months (not including at least three months of planning before hand!!). I was part of a team that performed a complete SAP R3 & NT 4.0 migration of approximately 1300 workstations over 30 sites within the calendar year.
I am sure there are instances where the phased roll out is required or preferred, as long as there is a plan for the roll out and a plan for the next upgrade, that's the important thing. Most firms plan for the upgrade they are paid for with reckless abandonment for the next upgrade - as a consultant one has to ask "Is it my job?" but we must remember the customer.
I'm rambling, so I'll stop.
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