Ahh yes very good point, there's many older devices+drivers that only support Direct3D properly, S3 chips for example.
But the same is now true if you don't use GL 3.2, it's actually the case that GL1 and GL2 legacy functions are increasingly being implemented in the drivers to use the DX11/GL4.x programming pipelines. As a result there's many instances where GL1/2 support is becoming unstable and even some legacy functionality (such as display lists) is not supported directly in hardware. The fixed pipelines in cards have long since gone and are emulated at the driver level. Nvidia tend to be really good with not breaking old things, ATI tend to break somthing in every driver release.
Intels latest line of chips are a good example, dramatically slower smooth line/point rasterization and will switch to software if you try to use line stippling (at least in my tests of 1 1/2 years ago). Any depreciated functionality could potentially suffer the same fate.
Remember they're focusing on game support and games that use GL1.x and GL2.x tend to be a lot less demanding on the card and thus they don't really care about optimizing that much.
So it's not a case of only switching if you need the new functionality, you should concider supporting just for performance/stability concerns with the same justification you rightly use for Direct3D.
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