A separate jointer and planer take up a lot of space. It's not just the basic footprint, both jointers and planers need at least 8' on both the infeed and outfeed side (10' is better, 12' if you can arrange it). For two machines, that's a lot of room. Sure, you can put machines on mobile bases and roll them into position, but that takes time. There's also the hassle with attaching/ disconnecting dust collection hoses if you go the mobile route. Don't get me wrong, you do what's necessary to make things work. In my previous shop, I had to roll the planer into position every time I processed stock, and the jointer too if the board was over 6'. That's something to factor in when weighing the overhead of converting the jointer to planer and vice versa: what's your overhead with two machines?
A lot has changed in the last 15 years in woodworking equipment. There are now several good machines that cost less than $2500, which in turn has kept the big boys (Hammer, Mini-Max etc.) competitive. But Bob is right about some of the space savings being "phantom". The Grizzly G0634 is a great machine (I've seen it in action!), but you have to remove the jointer fence to convert it to the planer. That means you need room close by to stash it. Also, the Grizzly is 43" deep because of the rack-and-pinion system. But it does perform very well where it counts.
I have the JET 12" jointer/ planer combination machine on order, the one with helical head that uses carbide inserts instead of traditional steel knives. This means no more fiddling with dial indicators :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers :icon_cheers (I stink at setting knives). The carbide cutters last a lot longer too. I looked at the price of a 12" jointer with helical head and a separate 15" planer with helical head...almost twice as much. The JET is only 31" deep, and the fence stays on during conversion.
Cranking the table up and down is definitely the most time consuming part of the conversion. I saw a modification somewhere so you can use a portable drill instead of cranking the table by hand, I'll see if I can dig that up. Also, if you forgot to joint a board and have to go back, resetting the planer to the exact same thickness is a challenge. One of those Wixey digital gauges is definitely in my future.
If I had a 30x40 shop, I'd buy separate units. No doubt about it. To me, the time to convert the machine isn't an issue, but there is definitely something attractive about not having to "fiddle" with things while you're cranking out stock. But this combo gives me terrific jointing capacity, a helical head for both functions, and space savings to booth. I'll let you know in 6 months how much I like it