My old MacBook Pro is limited to 3GB. With an HDD it was awful trying to run the latest Adobe suite. With an aftermarket SSD it became more tolerable, but it's still a single tasking machine. You don't want too many things open all at once. A lot of things worked against it though. Slower ram, much slower CPU (2.16ghz c2d), and both a small HDD and small SSD, and SATA3 bottlenecks on the internal drives. Neither technology particularly likes being filled past 50%-66% capacity unless you want to constantly repair discs to fight slow down.
New i7/i5 based machines are surely much faster, their SSDs aren't nearly as bottle necked and they have faster RAM, but they still have relatively small capacity storage. Something the average user will fill up in a hurry, and which will slow the overall system down markedly, though at least these can be changed later.
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