1) reason not to do it :
Most MSIE users switched to Firefox now and Firefox has :
a strong pop-up annoyance policy set by default.
Assuming 50% of WWW-surfers do not know or are not interested in how
to toggle that feature On/Off or allow pop-ups by domain, well, you might
end up with pictures never seen.
What you could do is give an explicit reason why pop-ups must be allowed on that site, all that along with the <noscript> tag that many tend to forget nowadays.
2) Do it, because, it is quite cool to have an image displayed with its own title, size, width especially with 'directoryindex=no,resize=no,scrolling=no, etc..', when one can click on the popped up big view to close it.
That kind of webdesign simulates a more interactive and personal catalogue browsing : user can choose pics and it gives him the feeling of a more neat browsing.
You want more jam & sugar with that ?
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FrankB.n00b()