Taking nationalism from liberialism against the ai, does not make much sense....Nationalism costs the same as Liberialism so you could tech it straight away and avoid the risk of losing Liberialism if it is a close race...if race is not close why not go for a more expensive tech like Astronomy or constitution or steel.
Against humans nationalism makes more sense as you will propably will not have enough time to shoot for other tech...
All my comments are based on playing against the computer AIs. I profess not expertise in human vs. human games as I have never played one in CIV.
I already made arguments for nationalism (and reasons to take other techs) but a few more
1) It's more expensive than liberalism. Based on marathon beaker costs: arround 6900 for liberalisn and 8500 for liberalism.
2) By taking nationalism with liberalism you are unlocking 3 civics with 3 more 2 techs away (consitution and democracy).
3) You need nationalism for constitution, but if you can nab nationalism from a trade prior to finishing liberalism then by all means take it!
4) Nationalism combined with the cheap music get's you alot closer to military tradition and some serious "Ownage". Alot depends on what you need, trade overseas (astronomy) or a powerful land-based military (chemistry) or a nice mix (Nationalism).
A few more comments about what I see as the problem with "waiting" on liberalism:
You can miss out on a lot of backfilling by holding education and liberalism in trades.
You lose the advantage of powerful economical techs in FR and FS which boost beakers. If you hold liberalism for 3 more techs to get something like democracy or steel, you are missing out on a lot of turns that add 2 commerce per town (assuming you have a larger empire with cottages) plus an extra 10% to the overall beaker output (and extra happiness). Pennywise or pound foolish, do you REALLY need that high beaker tech for free if you establish a very strong commercial empire alot faster than the AI.
DO not underestimate the AI at higher levels. They can spread arround education real fast and they build beakers in cities to speed along that liberalism race if they are close.
Once again, this article is for close liberalism races and is meant as a strategy article.